Q: Mild thud sound when starting or stopping.

Are the jack and spare tire tied down tightly? Are the brake caliper slider pins working freely? Are you missing any muffler hangers? Are all the lugnuts tight?

and have you checked uni joints?

This is the moment when I feel completely lost, because car maintenance certianly isn't my field. I tried without the spare tire and the tools, but it still made the sound, so I think they are off the hook. As for the other stuff, I let my usual mechanic decide, and he always says he can't find anything unusual.

The second mechanic said something about worn differential (not sure what that means), and that was the reason he suggested a complete overhaul of the transmission.

Anyhow, I think spending thousands of dollars on a new transmission for this particular car is not worthwhile, so I'm going to keep driving until it no longer runs. I just hope it doesn't breaks down in the middle of nowhere. __ sol

Whoa!

The differential and transmission are connected by a driveshaft, but they are completely separate units on a RWD car like the miata. In other words, one can go bad while the other still functions perfectly. If it is a bad differential, that in itself does not indicate a transmission in need of overhaul, and vice-versa.

62,000 miles is very low mileage for either part to go bad unless they are defective or have been abused. I would check with the mechanic for clarification and/or take it somewhere else for yet another opinion. Is the car shifting normally?

Also, if your differential is going out, I would expect it to start making a progresssively worse noise at higher speeds rather than just a bumping sound at starts and stops. How long has the noise been occuring?

I am guessing that the problem is one of the more simple things that have already been suggested or something similar. It could even be a loose sway bar bracket.

Is there a miata group in your area? If so, subscribing to their mailing list and asking about a local shop they can recommend might be worth doing.

Good luck!

Pat

"solamour" wrote

It was a (only slightly) loose battery in our '90 a few years ago that made this type of noise. You would think the mechanic checked for that, but isn't he the one who wants to rebuild your transmission because the differential is going bad?

The car runs fine. Shifting up and down is no problem. It just makes this rather dull knocking sound when I'm about to make a stop at a traffic signal or when I get a green light and about to go. It's like the car is cracking its knuckle. Well, the car doesn't have a knuckle, but you get the point.

Funny thing is, when I make a really, really smooth stop, like stepping on the brake ever so gently, almost no brake at all, I can often prevent the sound. However, I can't make the sound go away even if I accelerate really, really slowly, most likely because it's not slow enough.

The noise has been around for a while. But it's not quite consistent; it comes and goes. However, for the last couple of weeks or so, it became slightly regular with varying degree each day.

I did find a local group

which Ithink is worth checking out. Again, thanks everyone for taking time torespond. __ sol

At 62,000 miles, that miata should have a long way to go as long as it has had decent maintenance and continues to receive it. If you were in Austin, I could point you to a mechanic that would find the problem, give you an honest answer, and charge a very fair price. I assume that there are people in the San Diego club that know of such a mechanic.

If it were my car, I would, in this basic order:

1. check that the battery is secure
2. Look for any possible loose item that may be bumping the sheet metal in the trunk.
3. Shake the exhaust and see if it is loose enough to hit either the bumper cover or anything underneath the car.
4. Check that all lug nuts are tight
5. Grab the rear sway bar,shake it hard and see if it is loose at any of the four mounting points.
6. Check the driveshaft U-Joints for excesive play. This would be an unusual problem at 62,000 miles

This can all be done without taking anything apart. After that:

1. Grease the brake slider pins. This is a very good idea to do once every year or two anyway.
2. Check overall brake condition. Make sure that the pistons are not sticking in place at all.
3. Scream at the car - this helps, mine often cooperates after I threaten to strip it and sell the parts on Ebay.
4. Check the differential, he may be right about that, but it can be fixed or replaced without ever touching the transmission.

I am sure that I left something out, I always do.

No matter what, I would find a new mechanic when this one wants to overhaul a transmission that is shifting fine because of a noise that is coming from the rear of the car.

Again, best of luck!

Pat