RPM noise in SOHC

[762mm]

Hi,

My 99 SOHC is making a hissing noise when the RPMs go up above normal idle, and it gets really noticeable above 2000 rpm. It sounds kind of like an electric motor going really fast and about to burn... it's a very high-pitched sound.

I had the cam tensioner and the timing chain tensioner redone at Ford about 6000 miles ago, so I doubt that's what it is... I replaced the IAC about 3 months ago (Ford part), I have recent NGK iridium spark-plugs and new cables. At first I thought it was an exhaust leak near the engine (like near a cat or something), but it seems like everything is solid and there is no condensation or fumes leaking from anywhere noticeable (the sound seems to be coming from the front area of the truck). I know for a fact that something is loose inside my cat, because I can hear it tumbling around inside there when the truck is in gear, but I don't think it has anything to do with the noisy RPMs. I also tested the battery to see if the alternator was working properly... it looks like it does, as the battery has 14.25V at idle.

One last thing is that I think I heard like a "bang" followed by rattling metal a few days ago as I was leaving for work... It sounded like something got knocked in the exhaust, but my windows were up and I didn't hear it very well.

Any input would be very appreciated... I have to go on an important trip soon, and I have to make sure that this thing doesn't die on me in the middle of nowhere. Thank-you in advance for your help.

Edit: Forgot to say that the noise happens at all times when I give it gas - In park, neutral, or when I drive it - I can hear it inside the cab... The high-pitch noise gets louder as the RPMs get greater. I just checked the engine fan too, and it seems to move with only a bit of resistance (doesn't spin, but moves by hand with the engine off. I assume that is correct and that the clutch is ok). Thanks again.

 

[762mm]

Update: I removed the new IAC and put the old one in, no effect. I played with the throttle while the engine cover was off and listened, and the sound doesn't seem to come from the fan/pulley or IAC area, but rather from the back (firewall) side.

I don't know if it's just me, but it seems like the truck is missing some power when moving on from a traffic light or stop sign. Also, I test drove it on the highway, and the "O/D Off" light either started flashing or was flashing on the dash... I'm not sure if I didn't accidentally hit the O/D button though earlier. When I got to my destination, I turned the engine off, restarted it and the O/D light was gone. I drove back home on the same highway and tested diffrent speeds, but everything was normal.