Nasty shaking and rattling at idle.

First things first, when was the last oil change completed? Old oil breaks down in viscosity and is more readily known when the temps are up and noisy lifters can often be remedied with a simple oil change.

If it is your valve train that is at fault then it could be a number of things that can cause that kind of noise.

The oil was changed at the dealer that I just purchased from. My mechanic says the oil pressure at idle when hot drops to 22lbs. 115K. Reasons for the drop??

Chances are then you may have a faulty oil pump and this needs to be checked immediately before any further damage is caused. If your mechanic saw the drop in pressure he should have told you this already as the noise is oil starved components.