2015 honda fit ex manual transmission 6 speed. odd sound from left (driver) drive train section.

117k miles (highway). transmission grease swapped 3x so far engine oil changed relatively every 5k miles with proper oils and products

I have a sound that's happening that several mechanics haven't duplicated nor identified that seems prevalent after the car has been driving for a while. doesn't show up when cold

At first I thought it was a cv axle thing but now I'm wonder if it's a transmission thing. so I want to test with a thicker oil to see what happens. what's the thickest manual transmission oil I can safely put into the transmission of this vehicle? I understand people will have opinions and "i wouldn't because . . . . " but I really would like to know the answer to this question especially since the honda mechanics all just said the usual "honda mtf recommended oils". and I still don't know what's wrong with it
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it started out sounding like something flapping on rubber when it was in low speeds and I noticed it more when turning left on an even grade. then as I continued to try to find what the mechanics couldn't I noticed it happening when driving straight and no stress on the drive system (coasting, etc) and on an even grade and sometimes I think it sounds mechanical or metalic. and turning sometimes reduces the sound (after it's warmed uo)

on short drives after the car is cold (less than 15 or 20 miles) I don't hear it.

I also notice when it's happening that after coasting then applying brakes, it seems like some slack is taken up like a very slight "clunk" the same when after stopped and I am starting to move again

when I put the car on stands and try to test everything myself the boots look fine and the only "play" seems to be where the cv axle plugs into the tranny/engine side.

oil thickness and suggestions what else to try for diagnosis greatly appreciated.

 

I have no answer to your question re: trans fluid, but....could it possibly be your wheel bearings? Just throwing that out there

 

anything is possible but I'm not seeing/hearing anything that would make me think yes, and I would have thought that the mechanics would have caught that, and I wouldn't think that cold vs warmed up would mask it, but again I don't know so I'm still trying to figure it out

 

Flapping rubber: Have you checked the splash guard / skid plate, whatever you want to call it? The clips at the back are notorious for coming loose, and if that happens at speed, flapping noise and vibration up through the console. It wouldn't be an intermittent problem fixing itself, tho....just something else to rule out.

 

I'll tell you what you don't want to hear... Don't try to hide the sound with thicker fluids. The sound will still exist if it's the transmission, and you'll simply be deadening the sound with heavy fluid. The deal with heavy oils is that you are basically taking something that you think may have a problem and then ensuring that you for sure have a problem.

It sounds like a wheel bearing but also sounds like a control arm bushing in another part of your explanation.

You mentioned coasting.... Try different scenarios.
1. Coast in neutral
2. Coast in gear
3. turn in neutral
4. turn in gear
5. deceleration
6. acceleration
7. deceleration while turning
8. acceleration while turning
9.... you get the idea...

Take notes if you have to. Then see what the most common scenario is, like turning vs braking vs slowing down.

 

Be very very careful about dinking with the oil in a transmission, modern transmissions have a low tolerance for mis matched viscosity or chemical packages, there are different materials the synchros are made out of and getting the wrong stuff in there can be terminal to the transmission. I grabbed the wrong stuff once, and ended up rebuilding the transmission on my jeep. GL-4 and GL-5,, not the same thing..

 

I apologize for the delay in responding. dealing with multiple things. thank you for the suggestions on troubleshooting.

And for the cautioons. Again I understand but for a manual transmission using a thicker oil "that is safe" does multiple things.

1. thicker is better for performance (just) to a certain extent just like thinner is better for shifting (just) to a certain extent.

so even if it were to remain in the transmission a little longer than simple diagnosis it would protect the transmission provided that "it is safe". that's why I specified it being safe when I asked I apologize that I didn't state it sufficiently.

2. Diagnosis. if the sound is absent or notably less prevalent after the transmission warms up, then I know where to focus. if not, then I have spent maybe 20 to 30 dollars to rule something out versus several hundred for an axle. and I have more data than before that I can take to the mechanics.

So if anyone had any information they can share I would be grateful. I will continue to try to locate the issue and will continue to look at this thread and try the suggestions that I haven't.

Thank you all for any suggestions and information you can give

EDIT:
and inre the skid guard and mud flaps yes I checked those when it first started. They are all securely anchored