Noise in 3rd and 4th gears, sounds like exhaust leak
 

Thanks everyone for all of your advice, I read hundreds of posts before purchasing my vehicle and have relied on all of your expertise it has been invaluable. I have searched the site dozens of times for information regarding my problem but have come up empty, if it's buried in the archives please redirect me.

Purchased my 2010 GT with 6 speed about one year ago and noticed a problem that occurs about 50% of the time. While in 3rd gear you can hear a minor noise that is much more noticeable in 4th gear. My best description is that it sounds like an exhaust leak but only in those gears, I shift into 5th and it disappears. No power loss, no consistency (hot or cold engine, different weather), can appear one minute and disappear the next. Literally on a 30 minute drive it could appear and disappear 5 different times. And when it disappears it is silent and sounds perfect. The car has 31K miles on it and I just upgraded to BHR coils and new plugs hoping that might solve the mystery. The car starts much better and definitely runs smoother so thanks to everyone who recommended this upgrade. However, the mystery exhaust leak/noise persists. Appreciate any ideas how to identify what this might be. Thank you.

Off my head i can think of 2 possibilities.

Inspect your gear boot for any holes, try to cover them and see if the noise goes away.

Put the car on the lift leave it running, and inspect the exhaust gasket it's usually sandwiched between the catback and cat, due to the heat produced by our cars it tends to get damaged over time.

Definitely not coming from shift boot.

Sounds like exhaust but the fact that it only makes this sound some of the time and in 3rd/4th gear keeps making me think it has to be transmission related

Hello maybe I can help :-) I think it sounds like a synchronizer jittering inside the gearbox. Or it could be even a spacer. The very first thing I would check would be the amount of oil that you have in the gearbox. I know on mine it was low when I bought it a little. And I know that I've seen this another cars. If it is making noise it doesn't mean that you've damaged it yet. If you continue to drive with low oil in the gearbox it will cause a problem. Try this pusher clutch in when it's making the noise and if the noise changes or disappears you know it's coming from the gearbox. My guess is it would disappear completely. This is an emergency so do it quick don't drive the car. You could end up with the synchronizer an overheated gear and the problem will stay with you tell you crack open the transmission. So Add the oil now and I probably will be OK.

Took it to the dealer, they checked the oil, the fluid was in great condition, no discoloration, no particles. Mechanic drove with me, he had trouble hearing the sound, but of course I hear it all the time since I'm the primary driver. He didn't seem concerned felt it was nothing worth tearing apart the transmission. Seemed like a reasonable guy, 25 years experience at Mazda. Frustrating because the sound is there or it's not there, nothing in between. I guess I will just turn up the radio louder to hide it until I get another opinion.

Any update on this? I am having the same issue in 4th +, but specifically in 4. Just had the fluid checked and it's fine, 100% clear and topped up. Living where I do, I only have one guy that knows the car so I'm limited in Mechanics.

You could be hearing transmission bearing noise. I hear bearing noise in 2nd and 4th gear on my 05 GT. Its something that I'll live with until I source a replacement transmission. I have had a few RX-7's over the years with noisy transmission bearings and they held just fine until I replaced the transmission. Just something to consider as a possible failure point.

Update is that nothing has changed. The sound comes and goes and it is mainly in 4th gear for me also. Performance has not suffered at all it's just strange that it happens so intermittently. The car is in storage for a few months (cold and snow are not the RX-8's friend). Maybe when I get it back on the road it will have gone away. Wishful thinking.