I have a 1998 F-150 4X4 with a 4.6L V8. The engine has a moan at low rpm. Ford has done a few things to it, but nothing helped. I understand they all do it, but it is very irritating. I would change the air intake system if I thought it would help.
P.S. I've been a mechanic for thirty years so I could try most anything.
Thirty years? Your hands must be in worse shape than mine. We haven't heard of any intake related noises on the '98 F-150. But did your dealership's service department try clamping off the heater inlet hose while attempting to diagnose the problem?
Ford Technical Service Bulletin # 98-12-12 deals with a "moan" or "hooting" sound on acceleration or deceleration at 1200-2000 RPM. The sound is actually caused by water pump pulsations which travel through the inlet heater hose and into the passenger compartment. If the moan disappears with the inlet hose clamped off, then replace it with updated Part # F85Z-18472-AB. The new hose should dampen the pulsations and eliminate the problem.
Another possible cause of your moan is engine vibrations being transmitted through the exhaust system to the truck's chassis. Careful alignment of the exhaust system is crucial in eliminating a "binding" condition which prevents the exhaust from hanging freely and creates a path for vibrations to travel. Ford is in the process of releasing an updated procedure for improved diagnosis and repair of this type of complaint.