Engine sounds like diesel at idle

I have a 2004 Trailblazer LS with the 4.2L I6 and this problem just recently started and I found this video on youtube of someone having the exact same problem.

www.youtube.com/watch?v=ZxFkboGEKxk‎

This noise is only there in reverse and drive. If you put it in park or neutral it goes away. Also goes away if you give it some throttle while in gear.

Have someone check for a cracked flywheel or loose convertor bolts.

Thanks I should have mentioned it seems to be coming from the front of the engine compartment.

Ok if you can hear this in the ft of the engine take the belt off and look for loose pulleys.( doesn't sound like it though) You probably will not get this accurately diagnosed on the net, someone knowledgable needs to check this onsite

OK thanks I was sorta leaning in that direction. Thinking possibly water pump cause I do have some play in the shaft and that is with belt on. I do want to take belt off and see if noise goes away.

Removed belt and started engine and no noise. With play in the water pump gonna replace that and see what happens.

Spin all the other pulleys. Any that are rough or loose are also suspect.

Thank you all for the help. After a lot of research I have found that the noise is caused by the actuator on the exhaust camshaft. I also had a loss of power and have somewhat gotten that back by changing the camshaft actuator solenoid. Found that you check the solenoid for oil in the electrical connector, which I had, and also could pull it out to clean and check for plugged holes, which I also had. Seems to be a lot of 4.2s out there with this noise. Thank you again.

Your solution just goes to prove what I always say on the forums, noises are almost impossible to diagnose on the net.