New Rear Breaks are thumping?

No flat tire, yes I drove with them and they heard the noise, it happens when you apply the brakes not while just driving, it doesn’t happen when you back up.
After they lubicated it, it stopped for a few miles and then started again the next day. There was no rain or water it went through. It is not in the trunk.
This shouldn’t be rocket science, I would think an experienced, qualfied mechanic would have it figured out. I would not have to be going back again and again for a simple brake job, The car is only been owned by me, driven by me, no accidents, only has 38000 mi on it. Was not effected by the flood as I drove it out of the area 3 days before my home was flooded. This car was always in a garage till June of this year. It is well cared for and no I do not ride the brakes. That car is a gift from my late husband.