Sable drum brake noise

At 109K miles I had new shoes installed in the rear brakes of my 1989 Sable. The drums were turned on a brake lathe as part of the service. Ever since the work was done, the rear brakes make a thumping noise
proportional to the speed of the car when braking heavily. The brake shop looked at it and claims they can't find the problem.
Any ideas?
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Any one of these "ideas". Drums could be out of round. The return springs ( most likely not changed, but should be at each brake job ) are week. The drums not machined with the correct surface causing the lining to chatter. Cheap linings installed.

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Add to the rest.....Hot spot(s) on the drums that didnt "turn" out...

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If this is the problem, will I need to replace the drums? The existing drums have been resurfaced twice.

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Thanks for the suggestions.
This shop does a lot of brake work (Meineke), and the drums were turned twice, so I think the machining and linings are OK. They didn't change the springs, so I will try to get those replaced. If the springs don't do the trick, should I then get new drums? I'm not sure what the best sequence of parts replacement should be.

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The drums have to be measured to know if they need to be replace, just take it back and let them fix it for free. They really should you know....Just tell them ever since they where done they thump...
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Armchair observer wrote:

the reason why they cannot find the problem is that the dont know what they are doing.... with brake drums and rotors being so cheap you just replace them with new rotors now and dont turn anything..
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