I have an 1997 f-150 with 155K trouble free miles on it....however this week this has changed. While driving with the a/c on, I noticed a "knocking" noise. I switched off the a/c and the noise stopped. Furthermore, the noise would only occur when the a/c compressor was engaged. I made the 800 mile drive from Viginia to Florida without running the a/c thinking if the compressor was abount to lock up I did not want to throw the serpintine belt. In Fla, a local a/c shop said the problem wasnt the a/c at all but possibly the timing chain. The noise only comes after the engine has run for 2 plus hours and appears to be coming from the left side (when looking at the engine). The local ford dealers are to busy to look at it untill late in the week. No service mgr that I could find on the phone has experienced the same symptoms...any ideas? I got a long trip ahead of me back to Va on Sunday.

 

you might check the plugs on that side,one could be backing out and is getting ready to come out ,threads and all

 

Ticking was a bad choice of words...knock is more like it.

 

Mine has had an erratic cowbell sounding knock at idle for about 45,000 miles. I first noticed it after the timing cover gasket was changed at 35,000 miles. It doesn't get worse or better depending on a/c being on or off. Is this what yours sounds like?? Mine seems to have no rythym, doesn't follow the engine idle. It doesn't seem to be hurting anything, so I just forgot about it. I asked around at an f150world get together, and they said these engines just do that??,,,,98

 

You also should check your a/c compressor mounting bolts, mine were loose and it caused all kinds of symptoms. Squealing belt, a buzzing noise on heavy acceleration, etc...etc...

 

Turns out it was the belt tensioner. Just as was about to leave it with a ford service mgr, I noticed the tensioner jump ever so slightly with each knock...whew! $49.05 for OEM part, 30 minutes for me to install it and the ol gal is as quiet as can be.

 

Glad you found it!! Stuff like that will drive you nuts,,,,98

 

I have a 1997 F-150 4.6L king cab Lariet with 94,500 miles on it. Only thing I have ever had to do is a rear seal. However, about 20,000 miles ago, when I turn on my air conditioner I get a rapid clicking sound. I looked at all the pulley and watched the belt motion and on the compressor it looked like it would skip while turning, almost as if the clutch in the compressor was not working correctly. I am getting no air right now, and the clicking is really loud. Now that summer is coming I am going to look at this a little more closely. Is the sound you are hearing a "Clicking" or A a "thud"?

Rick