My wife noticed the other day when we run the AC my truck is making a really loud ticking sound. Almost sounds like bad engine knock. When we turn the AC off the sound goes off with it. What could this be?

Just to ad my AC run really cold. Havent had any problems with it. The noise however just started. What should i check? Belts? Loose bolts maybe?

It's coming from the engine compartment? I have a really annoying ticking sound too when i run the ac at free way speeds, but it sounds like its coming from right behind my dash or the ducts...

Ya mine sounds like a REALLY bad engine knock at times. The other day we went for a 2 hour drive in the mountains. Stopped at a store and i left my truck running as my wife went in. I heard this god awful knock and was talking to my son about how the POS gmc beside us sounded like it was on its last leg. My wife walks to the truck and says "babe your truck sounds like s*it" lol. Sure enough. I pop the hood and hear this god awful sound. Turn the AC off and the sound stops. I`m uploading a video to youtube atm but i cant replicate the loud sound however when i start my AC my belts scream!

same here it just popped up this summer, and it comes and goes.

Sounds like your AC clutch is going out. There are metal fingers that hold the flywheel to the ac pump shaft. Maybe one of them have broke.

Spin your compressor by hand and feel it for smoothness. If it is notchy and rough turning, then you compressor is crapping out. If it is smooth turning and quiet, it is probably your clutch.

Can you point out the compressor for me? I`m looking to turn this thing right?

98 4 runner AC compressor - Google Search

Mine does the same thing. Are you asking where the compressor is? Passenger side bottom of the engine compartment.

I have a ticking sounds that is comming from the a/c blower fan. It sounds like a piece of debris or something is in the housing. It comes and goes.

I dont think this is what your referring to though...its nothing like a loud knock from under the hood.

I have the same ticking/squeeling from the blower fan at the passengers feet (started as intermittent then got maddening). Took it out and cleaned it the best I could but no luck, I actually made it worse. It's pressed together so looks like I'm just going to have to order another one. Let me know if you fix yours.

Passenger side of engine, under power steering pump like mark3406e said. Seen more easily from underneath, like so, the more prominent thing in the pic with a belt.

Sorry about the mug, I was finishing up my timing belt job and my hubby snuck under the truck to take a pic.

I have slight knock at idle in gear warm and it seems to be related to the A/C compressor. Sometimes it's there and sometimes it's not so I figure can't be anything too serious yet. Is annoying though. Reminds me of one of my old Ford Falcons (1961). Darn thing sounded like crap all the time.

Ever get down to the bottom of this?

Thread revival time…

I’ve had the same issue as the OP for the last few years ~ 60k miles. Idles perfectly fine, but with the AC compressor running there’s a fairly distinct taping noise that varies in loudness and the frequency increases and decreases with engine RPM. When the motor revs high (above 2500) the sound can’t be heard and the engine runs great. The sound is loudest toward the bottom end, oil pan area, and I can feel a light tap on the shifter (she’s a manual) when at idle with the AC on.

New OEM compressor, idler pulley, full tune up, belts, etc didn’t fix it.

Over the years it’s had timing components replaced, new alternator, all pulleys. I’ve checked the crank key (looked great), and replaced the harmonic balancer. Clutch as well… probly more also… but the problem has persisted.

Eventually I had a reputable mechanic take a look and they never were able to put their finger right on the problem but after checking out each component they deduced it could only be the front main bearing going out. They said every component looked great, but they could change the sound of the tapping by changing the accessory belt tensions.

Soooo… OP could have a bad main bearing like mine, likely caused by over tensioned drive belts.

My guess is the slight load on the motor at low RPM increases the loudness of the tapping. I’ve been waiting for it to get worse but it hasn’t yet.

Not the most uplifting info for anyone with the same symptoms, but hopefully helpful.

Man, I searched hi and lo for that dam noise and my lovely wife reaches up and turns the fan down... ticking reduces.... Doh!

Now I know.

the stereo gets turned up.