Hey there fellas,

I created an account here just to get advice about this, so any ideas are appreciated. I'm at my wit's end.

My wife has a 2013 Mazda 3 sedan, that has developed an incredibly annoying rattle from the rear. It sounds like marbles in a can and does not scale with speed or engine RPM.

The rattle is most noticeable when driving ~20mph across slightly uneven surfaces such as roads like shown in the picture below. Just slight vibrations make the noise happen.



I have done the following to try to find it and fix it

• Took it to a shop that we like and have used for years, they couldn't figure it out, and they returned the car to us and said they were stumped.
• We removed everything from the interior of the car, including the trunk floor liner and all tools and stuff underneath. I then rode in the trunk and it sounds like the noise is coming from either both sides or directly underneath the trunk area. Hard to tell for sure.
• I removed the rear shocks from the car entirely and drove around. This didn't fix it, but it did "muffle" the rattle a bit.
• I inspected the rear springs, they are not cracked. Not even a little piece at the bottom like I've read is common.
• I removed both the sway bar end links and drove around. No change at all.
• I found a small stray heat shield up by the rear axle that I was sure was going to fix the rattle. No change.
• I went around with a flashlight tapping and hitting everything I could think of to try to find a loose bolt or bracket, or anything like that. I didn't find anything except what is described below.

The one thing that I did find, is that when i lightly tap on the bottom of the gas tank, it rattles way more than it should. It is hard to tell where this rattle is coming from. Almost sounds like inside the gas tank, or maybe on top of the gas tank. None of the clamps or bolts are obviously loose in this area, I checked for that.

I'm not really sure how to proceed without removing the gas tank.

Is this a rattle anyone has ever heard of before?

Can you access the top of the fuel tank from the back seat by chance? I know some fuel filters are removed like that.

A little background about myself: I'm a mechanical engineer. I've been tinkering with engines, cars, motorcycles, etc since I was a little kid. I swapped the engine in my own BMW last year after I blew it. I like to consider myself pretty experienced when it comes to this kind of stuff but I'm totally stumped and need some help from someone who knows Mazdas!

Thanks for any help!

 

If its a BL chassis and not the newer BM chassis, they can have issues with the baffles coming loose in the fuel tank. Does the noise change from empty tank to full tank?