I’m looking to buy a 98 f150 from a friend but it has a knock not a tick but a deep low knock sound on a cold start up when the Rpms are high and as soon as it idols down it instantly and completely goes away!!! It’s almost like a solenoid or something along those lines because of how it just stops right away I wanna as kinda wondering if the choke could be making this noise?

 

Truck is fuel injected so it won't have a choke. Some may be able to help but I think you're going to have to give a little more info. What engine, auto or manual trans, mileage? Does it knock driving when you accelerate and/or coast? Does it knock when you rev it in neutral on the up rev and/or down?

 

it is automatic 4 wheel drive with 160,000 it has the 5.0, there is absolutely no knock at any time other than when it is first started up and the engine is below operating temperature and idol is high as soon as the idol goes to the normal idol it immediately goes away, you can not duplicate it by revving the engine or driving it in any way

 

The only thing I can think of may be piston slap until the skirts expand as they warm. Possible exhaust manifold leak or broken stud. There is also a tsb for diesel tick sound but that's a catalytic convertor issue that happens even when warm at idle.

 

Could it be oil pressure not coming up quickly? Has it been beaten all its life and/or had bad maintenance?

 

98 F150 with 5.0 liter? Doesn't sound right. 4.6 or 5.4 if it's a V-8. Sounds like piston slap to me which is pretty normal for many engines with miles on them.