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Recently after having my car sit for 3 months I noticed a metallic thud sound wich can be heard in any of the two following scenarios:

-When braking only when im reverse (it only happens once for every time I depress the brake pedal). -When going from stationary in to reverse (only one thud when the car starts moving)

My vehicle is a 1990 Citroën Bx and this sound is never heard at high speeds or anytime the vehicle is not moving in reverse. The sound doesn't seem like it's coming from the transmission. I recently changed brake pads and rotors on all four wheels but the sound is still present. The sound is comparable to hitting a stationary object in sound and feedback.

  • odd question, but did you make sure all your caliper bolts are tight? I had this happen recently where one of the caliper bolts came loose and fell out, the caliper was moving freely and slamming into the rotor edge when I went into reverse – Ceshion Apr 18 '17 at 14:09
  • also check the wheel nuts as these also clamp or lock the discs to the hubs... – Solar Mike Apr 18 '17 at 15:04
  • Also, have you checked your fluid levels - particularly ATF? – kyle_engineer Apr 18 '17 at 17:02
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    Try to see if with someone's help, you can isolate where the thud is coming from. Drive shaft area, CV joints, wheels and brakes, perhaps the motor / transmission is shifting under load and there is a defective mount, etc. There could be lots of things giving you these symptoms, and additional data will help better resolve your problem. – mongo Apr 20 '17 at 14:32
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Assuming that the discs/pads you fitted are the correct type for the vehicle and that everything is tightened up properly.. The sound you are hearing could just be the pads shifting in the callipers​ as you switch the wheel rotation from forward to reverse motion.

I'd advise that you perhaps remove the wheels again and double check everything you did..

Does everything look ok? Is everything tight? Are calliper sliders/pins lubricated? Are any anti-rattle shims loose?.. If so renew them or try bending them so that they create a little pressure against the pads.

  • Some specific types of brake pads get glued to the calipers. Could it be that some failure there could provoke this behavior? – Martin May 19 '17 at 7:59

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