I’ve got a '94 Mazda miata which is in good shape, with among other things a fair number of suspension upgrades. It drives really well, other than the clunk. On the occasional bump it clunks as if something is loose on the driver’s side rear. I’ve checked the shocks, springs, jacked it up and inspected everything I can think of. I’m looking for wisdom from your decades of experience (in your terms wasted time) on what it might be.
Mazda Has Made Available A Part (# NA75-42-76X) To Solve A Thumping Noise From The Rear Of 94 - Early 97 Miatas.
The part, a pad, is installed to the upper front of the gas tank. It’s not necessary to remove the tank as enough access room can be had by just loosening the 4 gas tank mounting bolts. The area must be cleaned before installing the pad and retightening the bolts. Don’t burn yourself on the exhaust system.
Anyhow, check that area out on the Miata and see if that’s where the noise is coming from. If so, buy the pad and stick it on the tank. See if the friendly Mazda dealer will make you a copy of the TSB (Technical Service Bulletin - written for Mazda techs.) that outlines the location and procedure.
You might also check the spare tire and make sure it is secure in the trunk. A loose spare tire makes a lot of noise going over a bump, and you would feel it in the steering wheel.
I have no wisdom to offer for your Mazda, but have a story that might provide some encouragement. Many years and jobs ago my boss bought new a Fiat 124 that soon developed a heavy clunk coming from the front. The noise occurred infrequently and was seemingly random. The dealer couldn?t duplicate the noise and sent him on his way. The boss asked me, probably because I was the lowest paid guy in the shop, to see if I could find the problem. For the next several days when I had time I drove that car every which way trying to duplicate the noise. It occurred once under heavy braking, but not again. The first clue came when I was rolling backwards on a hill and hit the brakes. It clunked. At the next stop clunk again. Back at the shop I got under the front, one guy got in the driver?s seat to apply the brake, two others pushed the car back and forth. The source of the problem immediately became apparent. The front cross member bolts had loosened just enough to allow it shift against the frame. Pull forward and brake one clunk as the cross member shifted. No more clunks until you backed up and hit the brakes shifting the cross member back. The key was taking the time to discover the conditions that created the noise then finding a way to duplicate them. It also helped to have a lot of manpower available and a little luck. Best luck finding your clunk.