Hey mate, I recently had the same problem. I have an 01 SRi and I ordered new discs and pads.
They sent me the wrong pads for the rear. Apparently on this model there are two types of rear calipers, Bosch and Lucas. The minority have the bosch calipers, which are actually the same as the four stud non-ABS model (they're about the same size).
If you can actually move the pad in the caliper, then this maybe your problem. Once I got the right pads, they were a very tight fit.
Another possibility is that they have the right pads, but mixed up the inner and outer pads.
Another thing to check is if they re-fitted the anti-rattle shims/plates. These are the metal plates that sit between the caliper and the pad for that very purpose, to stop them rattling.
If you do have the wrong pads, you will have a very loud clunking noise. Because mine didn't start rattling for a while, I initially thought it was a driveshaft, that's how loud it was.
If all that checks out and there's only a slight rattle, you could use some anti-squeal compound (it's like a red silicone), but this will only silence minor rattles/squeal.
I would take out the pads and check everything. It's a pretty quick job and you don't have to push the piston back as you're not changing the pads (yet).
Hope this helps