How to check the flywheel in 2008 avalanche?

Quick question, have you checked the oil level?

Motor oil in your radiator may be a sign that you have a blown head gasket or even worse a cracked head. ---------------------------------------------------------------------------
lexia3 V47

I'm assuming your mechanic changed the rod bearings also. There is also a thrust bearing which keeps the crankshaft from sliding back and forth as it spins. Also check your flywheel/flex-plate, depending on whether your vehicle is auto or stick. Sometimes they crack and can cause a knocking sound.

if it gets louder and faster as the engine speed increases then a piston is knocking...if it is just at idle like you said then it could be a bad oil pump if oil light is on at idle or it could be the connecting rod knocking on the crankshaft because it is loose.

is knocking coming from the engine or transmission? does the knocking get louder when go faster? any lights on the dash on? what color smoke out exhaust? any wierd smells?

Here's what you don't want to hear...A loud knocking noise from inside your engine is generally caused by a crank or rod bearing...when caught very early, sometimes they can be replaced. however, once the noise is loud, the attending crankshaft journal gets "hammered" out of shape and the job becomes very expensive and in most cases more costly than engine replacement. There is one other item that can cause a loud knock...your flywheel. Bearing problems generally cause low oil pressure. If you have that, it's gone...if not, have the flywheel checked before sending the engine to the scrapyard.

A knock is not good. Does it change when you put the car in gear? Is it loud enough to hear at idle? If it changes in gear then it is a possible cracked flywheel. If not you can idle the car pull the plug wires 1 at a time to see which one changes the sound. This will tell you which cyl has the problem. If it changes pulling the wires then it is an internal engine problem, get ready to spend some money, sorry. Good Luck.

Replace "idle speed motor" (also known as idle air contrl valve).

It sounds like someone has been rough on the car. Your transmission may almost be shot, or you need a flywheel repair kit. If you get a flywheel repair kit, you can save your transmission. A flywheel repair kit is cheap. But a transmission aint cheap at all.