Transmission or Engine causing this knock?

[kgarguilo]

While flushing my engines one at a time, I heard this noise coming from the transmission area. Sounds like a knock or maybe even grinding. It seems to go away or lessen in gear but the load on the engine is increasing, and the engine noise may just overwhelm the sound. It is hard to hear with both engines running. This was right after starting the engine so still cold. Also, I checked the fluid levels, and they appear to be in the center of the suggested range on the dip stick after the transmission and engine were up to temperature at 155ish degrees F. then turned off engine to check the fluid level. Looked at Android App for Fox Marine app for my Engine gateways and nothing seemed wrong but, the app does not present a full set of data like the Rinda Diacom, but no errors are logged, While I did not have time to do further tests, I thought maybe someone in this group would have an idea of next steps beyond the below "next tests".

1999 Carver 406 w/
1999 MerCruiser 454's with 1k hours Port Engine SN OL303934
Velvet Drive Transmissions, Model 20-01-005 direct drive - 2.455:1 ratio

Just spent 85 hours of run time down the TN River system to Mobile Bay AL 3/27/25 - 4/6/25 with no issues

One Drive link to Video file with audio.
vhttps://1drv.ms/v/c/9f1d79bbadce17c5/EX5EtKJ3xiNNhOo3y5MD_RsB4o8OY3Gn61kqTuyju5B8EQ?e=iu0HnC

My next tests will be:

Put on Rinda Diacom diagnostics software and capture all data to engine running info
Take the boat out into open water - Go below and listen while under way

[mjk1040]

A miss firing engine will make it seem as there's a transmission issue. A mechanics stephscope my help isolate the origin of the noise you heard. Did you notice any flucuation in the tach or the synic of either engine on your trip? Have you had a tune up done lately? Pop your distributer cap and inspect it inside, look for any corrision. Hopefully you'll find your issue!

[kgarguilo]

Source of the post A miss firing engine will make it seem as there's a transmission issue. A mechanics stephscope my help isolate the origin of the noise you heard. Did you notice any flucuation in the tach or the synic of either engine on your trip? Have you had a tune up done lately? Pop your distributer cap and inspect it inside, look for any corrosion. Hopefully you'll find your issue! :beergood:

Thank you, my tachometers in both analog and digital via Fox Marine Engine gateway are rock solid and synced easily when I last went out two days ago. I have not done a tune up and have had the boat 2.5 years, I bought her with 799 hours and now at 1082 hrs. No real records on any maintenance before I purchased. I will add inspection of distributor cap this week and will report back all my findings. I will also look into a mechanics stethoscope as well. Thank you so much for sharing your knowledge.

[Viper]

Ya you need to make sure the engine is running properly as it will affect transmission performance and cause all kinds of noises back there. As mentioned, an engine miss can cause the tranny to chatter. Too low an idle will do the same thing. A bad damper plate on the engine will also cause noises back there, so will a broken baffle in a nearby muffler. Noises in an engine compartment can be deceiving and easily mistaken to be coming from the wrong piece of hardware. Don't condemn any hardware until you're absolutely positive you've found it to be the source of the noise, even then, as in a transmission, the noise could be due to external influences.

[bud37]

The stethoscope is a good idea for isolating , if you don't have one, a broom handle, plastic pipe, long screwdriver all held to your ear can produce good results. its amazing what you can hear from the inside of anything mechanical, a little practice is good.

My guess from the rinda would be watching the knock sensor but even that may not produce any pertinent info....just try to isolate the sound first, if possible.

[g36]

I get some chatter on my velvet drives at idle. Been doing this for going on 13 years. Increase idle a bit and it goes away. Ive stopped worrying about it. I'm wondering if you have had this and just now noticing it. I also have t h e fox gateways on mefi 1 crusaders. Not sure what mefi you have but on a mefi 1 I don't really know what other info would be on the data stream even hooked up to rinda. If there is I wouldn't mind knowing. I wondering if you really have any issues especially if both gateways are basically showing the same data. If you do a tune up then you'll a least be starting with that knowledge of maintance done.

[Viper]

Ya a tune up is where I would start, doesn't sound like that's been done in a while.

[kgarguilo]

Thanks to the many good recomendations. It is a new noise for sure. I will do what I can this week and report back.

[kgarguilo]

Source of the post I get some chatter on my velvet drives at idle. Been doing this for going on 13 years. Increase idle a bit and it goes away. I'm wondering if you have had this and just now noticing it. I also have the fox gateways on mefi 1 crusaders. Not sure what mefi you have but on a mefi 1 I don't really know what other info would be on the data stream even hooked up to rinda. If there is I wouldn't mind knowing. I wondering if you really have any issues especially if both gateways are basically showing the same data. If you do a tune up then you'll a least be starting with that knowledge of maintance done.

Actually, I have MEFI 3 Fox Marine engine gateways. While the Fox Marine Android App has some of the dataset, it does not have all the data that the Rinda Diacom software displays. However, I think the only thing I will get that will be valuable info is; Knock sensor data as bud37 suggested, maybe ACM data at idle and actual idle info. I will likely try raising my idle through throttle as g36 suggests (to test the theory idle low) may be the issue. Today, I bought a stethoscope as suggested by mjk1040 and others from Northern tool for poking around as well to aid in finding the noise.
Viper suggested

Source of the post A bad damper plate on the engine will also cause noises back there, so will a broken baffle in a nearby muffler.

The mufflers are less than one year old and sound awesome by the way.

As always, I will share the outcomes. I will try to get down to the boat Wednesday or Thursday. Thanks everyone for jumping in to help.

[kgarguilo]

UPDATE: I went to the boat today and was prepared to go through my whole list of items to check recommended by many. I started with the automotive stethoscope, because it was easy. As soon as I touched the transmission, I realized the noise was not there, great tool. As Viper said, "Noises in an engine compartment can be deceiving and easily mistaken to be coming from the wrong piece of hardware.". Words of wisdom for sure! Next, I raised the idle with throttle control to 800ish RPM, the noise is gone, I lowered the idle to 750ish and still gone, then around 650ish the noise was faint but could hear it. My idle sits around 600 (592-615) and was dipping down around 592. See screenshot below:

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It appears as G36 hit the nail on the head when stated "I get some chatter on my velvet drives at idle. Been doing this for going on 13 years. Increase idle a bit and it goes away."

I will say, I think my idle is a little low, I will likely do a tune up (Plugs, wires, Rotar and Distributor cap) as suggested by a few and also replace the MerCruiser Idle Air Control Motor - part number 18-7697 as well. Hopefully that will raise my idle a bit.

Any thoughts on my idle rpm @ 592-615?

Thank you all who jumped in quickly - I am sure glad is only a low idle causing the chatter.

Summation of things to check, just in case someone else has the same issue.
• Look at and inspect distributor - mjk1040
• Tune up- mjk1040
• Stethoscope- mjk1040
• Too low an idle will do the same thing. Viper
• A bad damper plate on the engine will also cause noises back there, so will a broken baffle in a nearby muffler. Viper

Boat specs:
1999 Carver 406 w/
1999 MerCruiser 454's with 1k hours Port Engine SN OL303934
Velvet Drive Transmissions, Model 20-01-005 direct drive - 2.455:1 ratio