97 f150 hums

bought a used 97 f150 with the 4.6 v8. it's got aprox 125000 on the rebuilt motor and transmission. the only thing i've noticed is that when i push on the gas pedal there's a slight hum. it's only when i push on the gas, when i let off it goes away. it doesn't seem to affect the performance, acceleration is fine, no missing or lagging, just the humming sound. any ideas on what to check? there's no vibration associated with it, all the fluid levels check good and everything seems to be fastened securely. don't know if this will help, but this was a service truck and the body has 298,000 miles on it. also, the tires are inflated properly and have good tread. thanks for any help on this.

Geese! U-joints?

doesn't seem to be a u joint hum, i had that in my old truck. it seems to be coming from the engine compartment. i was wondering if anyone else has this with their truck. i'm still getting used to new noises from a new (to me) truck. my previous one was a 1990 f150 4X4 that i basically wore out. i hated to get rid of it, but i couldn't pass this one up.

Does it change pitch with vehicle speed or with engine speed or neither? If vehicle speed, it may be in need a new wheel bearings. If with engine speed, remove the serpentine belt (just need a socket wrench to relieve the tension on the tensioner) and start the engine. Do not run for more than a few minutes. If the humm is no longer there, it is an accessory. Most likely the power steering pump or alternator.

it doesn't change with either vehicle speed or engine speed, that's what's so weird. it's just a constant level hum whenever i push on the accelerator. no matter what speed the truck is driving or what rpm's i'm at. i'm going to look over all the linkages again this weekend to see if there is something i missed that's loose.

Stock air intake? If it is, remove the filter temporarily and see if the sound changes. If it has a CAI, it could be just that.

Maybe a fuel line is contacting the firewall?