Hey. Gots a '97 F-150 XLt, with a 4.6, 4WD, shortbed extended cab, AC, loaded. It makes a wierd vibrating noise from under the hood, sounds like the plastic over the engine. It does this til about 35 MPH, then stops. Doesn't do it at full throttle. Started after I replaced all of the sparkplugs and wires december before last.

 

It's pretty likely that either you didn't get something secured or that a spacer or isolater of some kind was omitted when you put it all back together.

Start taking stuff off and checking to see if the noise goes away.

Steve