Here is a how-to for valve cover removal. It's for the 2.4, but 2.2 should be very close.
https://www.chevyhhr.net/forums/how-...er-2-4-a-8554/

Surprisingly, I couldn't find a how-to for air intake removal.

x2 to what donbrew said about replacing the timing set, high degree of difficulty.

 

Ok, let's say it is the timing chain that is causing the issue. What would cause it to sound normal when first start up and then not when warmed up?

 

Usually it's the other way around. But, the answer is heat expansion.

 

I thought if the timing belt were compromised it would be bad as soon as you start the engine, not just when it warms up.

 

I used the term 'diesel' in the search window. Looks like some people have speculated that it could be injectors. It is notoriously difficult to diagnose noises from a description. The flip side of that, is Murphy's law that it won't act up while at the mechanic.

 

Engine internal parts can be intimidating, but easy if your do some research

A general guide to what your are about to encounter




A written guide with direction as to how to proceed with the procedure!


Timing Chain Service Procedures GM 2.2L L61 Ecotec, Camshaft Sprocket

 

I stand corrected, I have 120,000 miles...