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I've noticed especially on these 90's era Ford trucks they make a rather distinctive humming noise, especially when idiling, or turning slowley... Never really noticed if other trucks from this era make that noise or not... Anybody else know what I'm talking about? A friend of mine thought it was the power steering, just kinda curious as to what it is....
Are you talking about the buzzing type noise? A noise similar to the noise the truck makes when you turn the key from the OFF to the ON position? If so, it is most like your fuel pump, they are very noisy on these trucks. The sound the truck makes when you turn the key to the ON position is the pump priming itself...
its possibly the idle pulley... ive noticed that sound myself mostly around 1000rpm... kind of a low hum like you described... thought i was low on power steering fluid so i added some until it was full (i was a little low) but i still got the noise... my dad put a stick near the altenator and the pulley to "feel" the sound and he felt it from the idle pulley..
The sounds comes from the power steering pump, the reason is that most people don't know that Ford never spec'd power steering fluid for use in them. EVERY owners manual I've come across says to use either Type F trans fluid or Dexron III/Mercon trans fluid. If you drain the system completely and refill with mercon the noise will still be there but a LOT less noticeable.
Actually there is a very noticeable difference in viscosity between PS fluid and Trans fluid. There are actually more friction modifiers in trans fluid than in the PS fluid.