It is a chevy chassis. Started getting this noise, most noticeable at idle. I thought maybe the fan blade was hitting something, but now that I remove the inside cabin engine cover, it seems to be coming from a belt driven accessory, sounds like maybe bad bearing. Feel really dumb, but I am not sure what it is. It is directly center and top. I know down on the right is the alternator, and I think the one down on the left is the AC (is longer and has about 1/2 inch aluminum tubes coming out the back which seem to head to the front under the heater box). I assumed this one is the power steering, but it has something attached at the back with an electronic sensor attached and a large tube that runs down into a manifold type box that I think has tubes into the engine. I am baffled.