Manual transmission rattle

[mirageman]

Hello all. 2009 Yaris 5-speed. 322k miles. Yes. 48+mpg, burns a quart of oil every 8k miles, so I want to keep it. Rattles on acceleration 35-60mph. It is not a wheel bearing.

Long story short: There are threads on this topic here that seem to only say it needed a transmission. I was hoping someone knows what the actual issue was inside the trans so I might decide to dig in and do it myself.

Short story long: Past 2 weeks or so I've noticed a mild rattle after 35 mph up to like 65 after which general wind/window noise seems to muffle the sound. Happens only on acceleration. I was hoping it was just a wheel bearing but I did some testing today and determined it is my trans. I just did a trans fluid drain which was due, and got 2+quarts out, so no loss of fluid and very minor sparklies, which I expected as I have ground gears by accident several times over the years. I would expect much more if something was very bad based on the sound. I think the problem is in the differential. I jacked up the front passenger side off the floor, ebrake on, driver's front side and rear end on the floor. I put it in gear and went through the speed and heard the pronounced rattle. I left it in 5th gear, wife stepping on the gas and went to the wheel and heard no concerning noise at the spinning wheel. I DID hear the rattle in the center of the engine compartment in the back by the diff, though. I then got inside the car, stepped on the clutch and shut the engine off. I heard the rattle still, all the way as the wheel was winding down, so it is NOT the engine. I then put the passenger side on the floor, raised the driver's side and did the same and also found no bad noise at the wheel, but again heard the rattle in the center of the engine compartment in the rear by the diff. I did this standing in front of the car because I did not want to go under it (chicken). This shows either a differential problem or BOTH inner cv joints have decided to wear at the same time. I also noticed something that might be a clue. I have done hypermiling whatever since forever, where on long downhills I shut the engine off and put 'er in neutral. I would then use 5th or 4th and slowly release the clutch to start it up again when I have to. On the road, when engine off, in neutral, there is no rattle, but when I start to engage the clutch to restart, it IMMEDIATELY starts the rattle, even though the engine has not started yet. Note that this is different from wheel in the air testing as I have done today. I've already driven 1000 miles like this, but it was milder and I didn't know it was a trans issue... I thought it was just a wheel bearing going, hehe.... anyway it has gotten worse and I am scared to do any major roadtrips until this is fixed. So just simply wondering if anyone might know what exactly goes wrong in these manual transmissions. Differential bearing? Most likely I think. Thanks.