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Transmission/Clutch noise question

'95 Mazda Miata with transmission noise that’s progressively getting worse.

It’s always had a little bit of a throwout bearing chirp since the last clutch change, 5 years ago now.

So recently, there was a rythmic low pitch scraping noise that was matched to the engine speed. It did not occur in 4th gear.

It’s now advanced to an intermittent noise on top of the scraping noise, that sounds like a metallic twanging of varying pitch. It’s kinda scary sounding.

Again, it doesn’t happen in 4th.

I was thinking the throwout bearing was finally on its way out, but it only occurs with the clutch engaged, pedal up.

Clutch disengaged, pedal in, and the noise immediately stops.

So my three potential diagnoses are the throwout bearing, the transmission input shaft bearing, or something internal to the transmission.

I plan on doing the work myself, with the caveat that if it’s the third option, that I’ll just replace it with a ‘recycled’ transmission.

But I’d like to know how to definitively tell which is the defective part, or if I should just replace all the bearings while I have the transmission down? Assuming I can actually service the input bearing.

I have yet to drain the transmission fluid to look for metallic bits, in which case, I’m definitely just swapping the tranny.

If the sound goes away with the clutch disengaged, the noise is unlikely to be the throwout bearing. That usually makes noise when it is under load i.e. holding the pressure plate back.

Is this transmission a 5 speed overdrive? If so 4th is probably direct and there is no loading on the transmission bearings. As to which bearing is defective it would be hard to tell as the input shaft bearing, the counter shaft bearings, main shaft rear bearing, and main shaft pilot bearing are all rotating and carrying heavy radial load except in fourth. You may have to open up the transmission and visualize the bearings to ascertain which bearing is failing.

Sorry to not be able to help more.