Noise from transmission
 

W108.018 1970 280 SE M130.980 with manual transmission

Back from before my head gasket leak presented itself I had a noise that sounded like a grinding coming from the transmission when I would drive 5 minutes or more over 80 mph. When I depressed the clutch the noise went away immediately and completely. The noise would also come in when turning left. Again going away immediately and completely when depressing the clutch. I suspect a bad throw-out bearing.
Brad

Probably not the throwout bearing as depressing the clutch adds pressure to the bearing. When the clutch is depressed the transmission is disengaged from the engine and stops turning many internal components pointing toward something wrong inside the gear box if transmission related at all. Did the transmission have sufficient oil?

Good luck!!!

I'm really curious about this and I hope you post what you eventually find. The info about the noise being related to a fast speed run, OR a left turn, is very interesting.

Off the top of my head I would say it's not the transmission, because a transmission doesn't know which way you're turning, and it's not effected by turning, only the differential and rear axles are. The transmission is (IMO) such that if it had an issue, you'd hear it all the time because there's really no room for forgiveness inside a transmission. Also, why would "shoving" the throw-out bearing into the fingers on the clutch plate silence it? Interesting!!!

I think if it were mine, I'd check to see that I had correct free play of the throw-out bearing, and also make sure that the clutch hydraulic system was allowing the throw-out bearing to be returned to "not touching the clutch plate" position(like might happen if you had a partially internally collapsed clutch slave hose. Just talking out my rearend at this point, because I'm baffled!

The internal spring in the slave cylinder is behind the piston. The spring force keeps the release bearing in contact with the fingers of the pressure plate. The hydraulic pressure is released when the piston in the clutch master cylinder opens a relief (and fill) port. System clearance is established at the master cylinder push rod.

Many thanks for the thoughtful replies.
I have had the oil level in the transmission confirmed several times during oil service events.
The shift lever bushings on the transmission (the ones on the removable plate on the side of the transmission) were replaced along with the clutch and throw-out bearing were replaced when a recommissioning mechanical refurbishment was done several years ago.
As I have a either a bad head gasket or bad head gasket install from the same time and other evidence of shoddy mechanical work:mad:, I'll need to look into all aspects of the transmission. I'm hoping that if the the throw-out bearing turns out to be bad then I can stop there. If it isn't bad then I guess the next thing to evaluate is the clutch setup. I really hope I don' have to tear into the transmission. The car was competently serviced regularly until 15 years ago when it became a garage queen and there are only 80k documented miles on the odometer so I doubt a rebuild is needed.
I know if maintenance and repairs are done properly these machines are very durable.

Brad