Had the pads changed last week. Since then there's been a metallic rattle, getting worse, on small bumps. I'm taking it back but what could it be?

an echo ?

Might not be anything to do with the pads but they seem the most obvious cause (due to the recent work) – could be a sensor that is loose or something. Normally metallic rattles come from loose exhaust parts though so it might be an unconnected co-incidence.

EDIT: Is it just me, or does anyone else have suspicions that garages manufacture other faults when you car is in just to get more work out of you though?

Possibly, and very scary, the only things I can think of is either a) caliper hasn't been bolted back on right or b) the wheel hasn't been bolted back on right.

Brakes on cars are the one thing I ALWAYS do myself. At least I know its all back together again properly. I would give both rear wheel nuts a close look. If you can reach the calipers give them a shake and see if their loose.

Get it checked ASAP

Pulls up chair…. mine does the same. I've put it down to pads rattling as it stops as soon as the brakes are applied. Everything works/nothing obviously loose and it's not getting worse/better, so have put off looking until the pads need changing.

Broken spring,
PJ

PJ, it's not sitting low though, and feels ok.

Does the system have metal inserts (shims) between the caliper and pad (like MTB brakes)? sometimes those can rattle if the pads are releasing a bit too far. Long shot but try some really hard pumps on the brakes to 'pump them up' – it *might* stop the calipers releasing that little bit too much.

My rear right went on my Touran, felt fine to drive, every time the rear wheel hit a bump it gave a metalic rattle, thought it was broken exhaust hanger but they were ok, needed looked at on a ramp, supplied and fitted new spring for £102, my brother has had 3 replaced recently on his 2 cars, I blame the state of the roads!
PJ

Don't some pads have anti-rattle shims? Perhaps they didn't put them in?

Could be an exhaust heatshield or as mentioned a broken spring…..

The thread that’s back from the dead…..

Was finally bored enough to investigate ‘the rattle’ at the w/e. Turns out the inboard brake pad has an anti-vibration spring on the top of it to stop the pad wadering around. Whoever fitted the last set of pads put them in the wrong way round, forcing the sprung pad into the wrong half of the caliper which meant the un-sprung one was rattling freely in the wrong place. New set of pads and 45 mins later and the rattle is gone. Bliss 🙂