300D: Rear rotor backing plate bending, making horrid noise

[jhealey1967]

Hello all, I am new to the forums here and am happy to have found such a great resource. I have a 1984 300D in quite good condition which my partner and I have owned for approximately 1 year. We run her on Bio-Diesel from a local supplier and take good care of her. We have only one major gripe: the right rear wheel will occasionally make an awful, rhythmic rubbing noise at speed and when braking.

Here's what I know: When the problem first arose, my partner, who uses the car as her daily driver, told me that the car was making "wonky noises" from one of the rear wheels. When I would drive it around the block, however, I could not replicate any such noises. Within a day, she would admit the noise had inexplicably disappeared, and it usually didn't come back for weeks. At first, I thought she was nuts. But sure enough, when we took the car on a road trip, the sound suddenly came out of no where, and was truly awful. A loud, rhythmic rubbing/scraping noise coming from the right rear wheel. When I pulled over to look under the car and see what the issue was, I noticed that the backing plate on the passenger side rear rotor was bent outwards at the bottom quite significantly. I took what tools I had in the trunk and bent it back into place. It fixed the problem entirely.

Over the past few months, however, the problem has returned more than once. Each time, I inspect things and cannot find anything wrong that would directly implicate an issue with the wheels, rotors, or calipers. The pads are fine and have plenty of life left in them.

What I did determine, however, is that hard braking exacerbates the problem. Bending the plate back into place will alleviate the noise, but it might return weeks later after some hard braking on a curvy mountain road, for example (or if ignored for a day or so, the noise might just go away on its own, not returning for weeks). When I go under the car to inspect the problem again, it's plain to see that the backing plate is once again bent out of shape, and has to be massaged back again in order to alleviate the rubbing. Lastly, I have still yet to determine what it's even rubbing on--there's no scoring on the back face of the rotors, and no marks on the inside of the wheels.

What could possibly be causing this? Surely there is something I am overlooking. Why would the backing plate be constantly getting bent out of shape? Has anyone else ever had this issue? Thanks in advance for any ideas or suggestions!