Rattling noise/cold start

I have a rattle noise (almost sounds like timing chain) for about 3 seconds when you first start the engine. The sound goes away as it idles. I can cut the vehicle off, and restart and it doesn't make the sound again. But if I let it set for 5-10 minutes (i assume give the oil to fall back down in pan) it will make the same noise again for 3-5 seconds when I restart vehicle. I'm trying to diagnosis problem, and have read it could be possible oil filter. Which does have a cheap filter. We just received vehicle in our shop from a used car lot we do mechanic work for. And owner states he just had oil changed (to a thicker oil) to try and quieten the noise. Plus it is about a quart over full mark on dipstick. Before I start tearing down the motor and take the timing cover off to inspect, what do you suggest I could do before doing that? Changing the oil? And putting the recommended oil and filter on the vehicle? Could it possibly be the oil pump making this noise? Thanks.