I noticed if I turn on my defrost I hear a horrible noise coming from the engine bay. it's a loud grinding noise. Doesnt happen all the time. Anyone have any ideas?

 

Are you sure its the engine bay? Does it change with the blower speed?

 

Doesnt seem to change. I can barely hear it inside but outside is bad. Sounds like it's coming from up front.

 

Does it happen when you turn on the a/c too? If so it might be the a/c compressor clutch.

 

yeah I never use the AC. I tried it and it makes this noise and I can feel it causing my Jeep to vibrate.

 

Its probably the compressor going bad then.

 

Yes, the A/C runs in defrost. Auto designers decided to activate the A/C compressor when in defrost to keep the freon seals from drying out over the winter and having to recharge the system in the spring because the charge leaked out. GM has been doing this longer than most of you guys have been alive!

 

Well, it's nice to have the choice whether or not to run defrost with A/C (i manually rigged this up in a Subaru i used to own) but the other reason is that "defrost" can be used to de-fog the inside of the windows and you need the A/C for that. Never heard the theory above but interesting.

 

unplug the compressor and see if it still makes the noise

 

It's the compressor or the e-fan - As Dave mentioned "A/C" kicks on when the defrost is on and the O.P. indicated that the same sound occurs when the A/C is on.

There are no other moving parts and both the compressor and e-fan kick on when the A/C/defrost is on.

As previously suggested, unplug the compressor and re-try. When you start it up, leave the hood open and walk around to the engine bay and see if it's more clear where the sound is coming from.
You can also manually test the e-fan independently by jumping it directly to the battery and seeing how it runs.

There is a -small- off-chance that there is another engine mounted accessory (P/S pump, water pump, alternator, idler) that is bad and that's what's causing the issue (due to the extra resistance on the belt when the A/C clutch engages), but that's not likely.

Both the compressor and the e-fan are parts that are not uncommon to have a failure, so do some "noggin' pokin'" and let us know how it goes.