99 4x4 1500 5.9 , 150k miles

at about 2k rpm in first gear, and sometimes in second I hear this metallic buzzing sound, Tried to capture it with my phone but it couldn't pick it up.

Sounds like its coming from the front end somewhere, Can hear it with windows up or down.

Its louder when trucks cold, when its warm you can actually barley hear it anymore, Only happens at 2000 RPM, then immediately goes away. The sound is exactly the same every time.

Any ideas?

 

Loose heat shield somewhere. Check the cat first....

 

You think it would be a heat shield even though its RPM specific? Cat is brand new, sorry I forgot to mention that.

Does make sense though that it isn't as noticeable when engines warm....Metal expanding...hmmm..

 

That's what it sounds like from here.... being RPM specific kinda points to it as well, as you get to the correct harmonic frequency at whatever engine speed, and it'll buzz a bit. Check it anyway, and see if anything is sloppy.

 

Will do, many thanks.