I have a '99 f150 XLT 4.6, sometimes it makes a very loud humming sound, then I give it gas and it goes away. It only does it in the cold weather. I do have a K&N intake system, could that have anything to do with it? Someone help me out because it drives me crazy and I cant figure out why!!

 

I don't have a KnN system..but.. power steering pump maybe. Mine recently became very annoying. I'm getting looks at the stoplights like I got a supercharger on the thing..

 

If it was the steering pump, wouldnt it make the sound all the time, in the warm and cold? It is very annoying

 

If it sounds like it winds down after you kill the motor then it may be the pcv valve. While the truck is running put your hand on the pcv valve and see if it is vibrating. If it is, pop a new one in and the hum should be gone. Hope this helps

 

sounds like IAC valve sticking.. mine does it from time to time. ive cleaned it and replaced it it still does it.

 

A few people have experienced that problem after instaling the K&n, especially in cold weather. My freind had to take is out because of that.

 

i 2x the pcv

 

Maybe the IAC.

Does it do it once the truck is warmed up? Or does it do it even if the truck is warmed up, idle?

 

My 98 does the same thing and it's caused by loose catalytic converter heat sheilds.

 

Air silencer, Perhaps. Does this sound go away when the engine is revved above 1500 RPM in park?

 

I would venture to say both the PCV valve and or the IAC valve seen this before and after replacing those items it went away.

 

My 98 5.4L has been doing that for as long as I can remember , just after I shut down the EVR will humm for a bit - for about 8 seconds or so.. After I shut down the EGR valve closes (no more vacuum) and EVR vibrates and humms until the trapped air is expelled..

I don't think it's hurting anything , truck runs great, so I pretty much just leave it alone

 

Yea, once it does it, i rev i up a little and it stops

 

It still does it after it is warmed up, even driving it for all day

 

1. Try pulling your PCV valve and shake it , if it sounds like an empty paint can then it's good - if questionable , just replace it - it's only 3-6 bucks and this has been known to cause this problem.

2. Sounds air/vac related. Start your truck, let it run for 30 seconds, have your hood open before you start it. Shut the truck off and go look under the hood quickly - do you here a noise like air releasing?

3. Check the vac line behind the manifold at the 90 - this has been known to collaps/leak and could be making the noise at a certain RPM's.

Heres a pic of that line , the 90 by the firewall is the one that I'm talking about -