Mike, have the mechanic drive with chassis ears hooked up
If he’s an experienced guy, he’ll know what you’re talking about
Chassis ears lets you systematically diagnose these kind of complaints
He said back on March 5 post that at least one mechanic had already tried the ears but failed to find the cause. Doesn’t mean someone with more experience wouldn’t havea different outcome but the OP knows about the tool…
The chassis ears have six leads
It seems to me if the car is making the noise, you should be able to find it by using the chassis ears. If you need to reposition the leads and take another drive, so be it. I’ve been down this road before
db4690: Two mechanics have used the chassis ears, and neither could pinpoint the source of the noise. The first mechanic at an independent shop repositioned the ears three times, and test drove the car after each drive. It was at that point that I began to get exasperated. I thought for sure he’d be able to nail down the source. I’m wondering if the noise is coming from somewhere where they simply couldn’t hook up the ears. But thanks for the suggestion.
Mike, here’s another idea
At times, I have rode in the trunk of a car, while another mechanic drove. Crazy as it may sound, it actually worked a few times
For what it’s worth, I’ve also taken a couple of rides in the trunk of a car in an attempt to sort out a noise out.
db4690/ok4450: Unfortunately, we’ve tried that too. The Mazda6’s rear seats fold forward, so one of the mechanics lay back there. I’ve tried it with my wife driving as well. I can hold my ear against the bare sheet metal and the sound seems to come from directly above where the coil spring/shock would attach. If it hadn’t been inspected, I would have guessed a loose shock or loose shock mount. But that’s been checked multiple times. I’m wondering if the noise is at the bottom of the shock (again, maybe the control arm) and the sound is simply traveling through the shock to the upper mount area. I may try another mechanic with chassis ears again and hope for better results. But thanks for the suggestion…
I’m starting to suspect something less obvious, like a body/chassis seam that isn’t welded properly…
Turbo: That is one issue no one (including the mechanics) has mentioned. I may be taking it in for one final desperation diagnostic attempt, so I’ll mention that as a possibility. If this inspection fails, may be time to trade it in (as irrational as that might be). Thanks for the suggestion.
I think @TwinTurbo may have the cause, a broken spot weld would cause that noise and be very difficult to find. Maybe the ride-in-the-trunk with your fingers running over the shock tower area might locate the offending joint. Good Luck.
Mustang: Thanks, I’ll definitely try that tomorrow. I have an appointment with another (and last) mechanic tomorrow afternoon, so we’ll go through the various points raised in the discussion thread. I’m guessing a seam weld wouldn’t be one of the better scenarios assuming they’re difficult to get to. Thanks again.
Today’s mechanic thinks there is a loose baffle in the gas tank, which of course is an expensive fix (new tank, installed = $775).
mike
For what it’s worth, it’s quite possible “today’s mechanic” is correct
When I was at the Benz dealer, there were certain cars that also had loose baffles in the gas tank
When it got really bad, it sounded like something was inside, banging away . . .
Since most sane people wouldn’t weld inside a tank, it’s quite possible a new tank is your only fix
I think the fuel pump is accessible on this model by removing the rear seat and the access cover; an easy job.
If a tank baffle is suspected that should be easy enough to verify simply by doing the above and peeking into the fuel tank.
I’m a bit dubious that a solid clunk sound would be caused by a tank baffle though. Still wondering if it’s not a loose ball joint on the control arm though as solid clunk sounds are usually caused by a ball joint or shock mount.
Just some food for thought, but what about an exhaust pipe banging into the chassis?
db4690/ok4450: My hesitation with the gas tank theory is that the tank is plastic, but the noise sounds more ‘metallic.’ I could see how an older Benz with a metal tank might make that noise, though I’m not sure about today’s plastic tanks.
The Mazda dealer took out the back seat, removed the fuel pump, and probed around the gas tank and found nothing loose, though he admitted that he could only ‘reach’ about 2/3 of the tank.
I have one (more) last appointment with a mechanic tomorrow, so I have added loose ball joint to the list, as well as to re-verify the shock mount (in addition to control arm bushings). Many thanks again for the follow-up comments and suggestions.
mike
The tanks I’m thinking about were metal, but it was only a few years ago, and the cars were still under warranty . . . so I’m not talking about ancient cars
Plastic tank? With gasoline in it? You might get some mild thumping out of that, but not a “clunk” that anyone would describe as metallic. So I’m as skeptical as you about the baffle idea. I’d sooner go with the bushings.
So, what finally happened? Did someone find the cause?
Sorry for the late reply, I was out of town. Current mechanic wants me to come back so that he can drop the right rear shock assembly and triple-check the mount, the mounting bolt, etc. He agrees that it sounds like a loose shock. He said the lower control arm bushings tend to squeak and make other noises rather than a clunking sound. He also pried at the bushings with a bar and said the same thing everyone else says – all the bushings seem pretty tight. Will post back in a couple days when he pulls the shock.
I’ve heard of people with shocks broken internally that looked normal but made sound when rapidly compressed. Didn’t mention it before as it didn’t seem to match your conditions.
The other thought, your post on the tank brought up, was if someone tossed something into your gas tank- like a steel marble or something that barely fit into the hole. Kids love anything that resembles a storage place 
Plastic or not, a steelie rolling around in there would make a clunk sound.
One good thing about dropping the tank, it might make it easier to inspect the chassis/body seams up in that area where the strut mounts…