Trans Rattle when Hot & AC Compressor Kicks In
 

I've had this rattle for a while when the AC compressor kicks in. I had it looked at at the dealership a year ago or so and they said it was normal, though I'm not sure they were able to fully replicate it. So I pretty much ignored it for awhile but it seems to have gotten worse recently (with the summer heat). So I crawled underneath yesterday after a long warm-up drive, and the rattle is definitely coming from the transmission internals. It only happens when the AC compressor is running, which causes a noticeable drop in idle and the whole drive train to vibrate. And it only happens when everything is good and hot.

I'm no transmission expert, but to me it sounds like a bearing on the input shaft. I didn't have a friend handy to depress the clutch while I was underneath. This is the next test I want to do to see if the rattle goes away when the input shaft isn't spinning. I'm guessing it will go away. Could this just be a bad motor mount? I'm not sure if the drive train vibration when the AC compressor is on is excessive or not. Even if it is just a motor mount issue, I'm concerned that the trans internals could have been damaged by all the vibration up til now. Hence the rattle getting worse.

Hi, for interest sake how many miles/work has your S2 now done?

Anyway, IMO it would be good if you could get your friend to operate clutch pedal, just to try and isolate this vibration.

I have not got my 8 on the road at the moment, however, ac compressors can get a little noisy/rattly when they age (still work OK), the rattle/vibration can then transmit through to transmission/shifter.

As you know trans roll over gear noise when car is hot and clutch pedal out again is normal, input shaft rotating...it only takes about 5000 miles (roughly) from new and you will hear this rollover noise all the time at hot engine idle.

My Miata now does it, in fact I can't ever remember any Mazda with a MT which did not have some sort of gear or gear rollover noise, input shaft noise or throw out bearing noise, perhaps the old 4 speeds were pretty quiet, but they did not have large and gear loaded counter shafts and triple cone synchro's, etc, etc and extra gears.

IMO if it was an engine mount I would have though the vibration would be there without AC on (compressor running)...even if car and engine bay is hot.

Many do not know but the RX-8 AC Compressors (without mag clutch) actually originated in the 2003 NB Miata, then Mazda used them in all RX-8's with a different mag clutch/pulley.

I think your problem is compressor under hot load, I bet if you raise RPM a little (throttle) as possible it almost disappears?

Sorry I cant be more helpful.

Thanks Ash, you are probably right, it is probably normal. I'll try to get a video of it this weekend to post though.

I'm at 40k miles now by the way.

its normal. the higher load at idle causes more vibration since the shifter is a direct link shifter it will rattle more.

Just to be clear its not the shifter that is rattling. I climbed underneath the car and could hear the rattle coming from the trans internals.

if you are in neutral there will be some rattling sound. it sounds like a faint marble in a can sound. record the rattle you describing. I think you are just thinking too much.