My first thought was they've forgotten to put the drain plug back in, but even they'd have noticed that straight away.

I'm with Mark on the fact that the engine is probably OK. That said I would not drive it anywhere or start it any more if it's rattling or making different noises. Had a quick look to see if you can see where it's coming from?

Has the oil pressure light come on?

For an oil change they really should have only needed to touch three things. Drain plug, oil filter and rocker cover filler.

I'm now going to say oil filter isn't tight.

Without pulling down the engine, you probably really won't know until it breaks (if it actually ever does).