2018 MT EX. Got my Fit last March and it has been a blast so far. It was a little under 100k miles when I got it and I do quite a bit of spirited driving/autox so I had gone through and done all of the service to make sure nothing fails on me. Just before a big road trip last year in November, I started to hear a 'whoop whoop whoop' kind of vibration sound and I have narrowed it down to something with 4th gear. I have replaced the front and rear passenger wheel bearings and the sound is a little similar to that. It changes frequency with speed and goes away if I shift to 3rd, 5th, or push in the clutch. It doesn't feel any different than other gears besides for transmitting the vibrations into the cabin a little bit. I had replaced the transmission fluid with the recommended Honda fluid last year as well as some brand new OEM motor mounts a few months ago. When I drive around town, 4th gear is perfect for the speeds I'm going but I can't stand the constant low vibration and noise.

Has anyone else had a problem like this or have recommendations on how to fix it?

 

Sounds like a Mainshaft (input shaft) bearing issue. Looking at the GE Fit manual (GK is subscription-only): 3rd, 4th and 5th gears are on the main shaft. 3rd and 5th gears are selected by moving their respective collars towards the clutch end of the shaft. 4th is selected by moving the 3-4 collar towards the opposite end. The differences in end-to-end forces are triggering the noise - it's probably a bearing problem as that's what keeps the shaft from moving laterally (or any direction except spinning really). Take a quick drive to get fluid circulating, then drain to check for suspended solids in the fluid.

 

Got a chance to change the fluid out. The stuff that came out looked good still and there were no large chunks of anything in there. I did see maybe a few teeny tiny little specs of metal glitter, but it looked very insignificant in my opinion. Could only see them when I was looking really hard and it definitely wasn't scattered through the whole pan. Filled with Honda M/T fluid from the dealer and it feels about the same and still makes the noise and vibration in 4th gear.

I don't really feel like my clutch is ready to be replaced and I'd love to do all of that at the same time I service the other transmission stuff. Is there any risk in running it as is for now and just dealing with the noise? I assume it would start to get noticeably worse before it fails completely.

 

"Doc, it hurts when I move my finger like this." Doctor: "So don't move your finger like that."

If everything else seems normal, don't use 4th gear until you can get in there and inspect/replace stuff.