On my 1500 beetle, the left rear wheel makes a bit of rattling noise like the rim isn't tightened, or axle nut is loose.

I tightened the axle nut very tight with a huge breaker pole and it's definetly the right torque. If I push and pull the wheel it rattles (not much really), but the right wheel doesn't.

Any suggestions what I can look for?

I'd check the wheel bearings

yep sounds like a bearing is kaput

I'll have a check. That axle is always troublesome, I have replaced the bearings about 4 times, 5 boots, and replaced the axle as well. Admittingly, the last bearing changed was when the axle was replaced about 4 years ago.

Check the Axle is Straight while it is out
a bent axle will chop out bearings regularly

Stop doing burn outs!