When I have my AC on when my truck is in Drive, I hear a sound like a steady chirping of a cricket.

When I put truck into Park/Neutral, the sound stops.

When I have fan on without AC, I hear nothing.

New Ford AC compressor from RockAuto was installed by a shop less than 10K ago, AC blows cold.

What could this chirping sound be that only happens when AC is on and truck is in gear?

Thanks!

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What you are hearing is, I believe, the acceleration of the crankshaft as each cylinder fires. With a low idle speed, and the A/C engaged, the engine is working close to the power limit at idle rpms. The crankshaft increases rpm as each firing occurs, and slows until the next. The belt slips just a tiny bit when the crankshaft increases rpms, and causes chirps. Do not put 'belt dressing' on to make things quiet, as it will only increase deterioration of the belt. Make sure the pulleys are clean and free of debris or any lubricant. Also check the tension on the belt.
If all those are OK, not much more you can do but leave the trans in neutral when at a stop, or ignore it. Turning up the volume on the radio sometimes is an easy solution.
tom

Another fun trick is to take a bar of soap and barely touch the grooved side of the belt while its running lol, not sure if that is hard on the belt like belt dressing is!

I suspect it's the belt as well. And just for conversation's sake, when I believe i have belt noise, I like to put on a new one and toss the old one in the toolbox or trunk for emergencies. i do the same with windshield wipers hahaha! i'm weird

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1999 xlt 2.5l 5-speed Supercab 159k
83 diesel 2.2l na
06 F250 6.0l psd
{click for lol}

Thank you, both.

So, is the solution to put a new serpentine belt on?
That would be easy-peasy...

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I would clean the pulley grooves, all, with something that would remove any oily film. Brake cleaner might be good, but don't get it on everything under the hood.
I'd get a new, quality brand belt, such as a Gates or Goodyear. I don't have a lot of trust that the Whirly Brand Belts, imported from WhoKnozeWhere, are going to give me good service. You may find a deal on Amazon for the belt you want, and not spend as much, or go local, and get one from a local parts store. You can inspect their offerings, and find one that feels 'new' as if it hadn't sat on the shelf for a dozen years. The rubber(?) will look dark, almost wet, not dry looking, on a newly manufactured belt.
OEM might be good also, if they have good turnover of stock.
I figured long ago that factory wiper blades lasted so long because they never sat on a shelf, and were fresh from the factory, so had max life. Same for OEM factory installed belts.
tom

i pretty much use Gates belts, easy to find around here. never had a problem that i can recall

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1999 xlt 2.5l 5-speed Supercab 159k
83 diesel 2.2l na
06 F250 6.0l psd
{click for lol}

Okay, thanks very much, guys.
I ordered a new Gates serpentine belt...hope it fixes the chirping sound. Will update.

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So, it wasn't the belt, and I cleaned the pulleys too. I also spun each pulley, there was some random squeaking coming from the alternator pulley but nothing serious.

Does anyone know what else this cricket chirp sound could be?

To review, I hear it at idle, when the AC is on, and truck is in gear - but NOT in Park/Neutral.

Thanks!

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Random squeaking from the alternator pulley is not normal. The armature should spin without noise. Only two things can cause a squeak when spinning the armature, the bearings or the brushes. The brushes ride on 'slip rings' that make constant contact with no interruptions such as you see on generators and small electric motors. They should be noiseless. The bearings are ball bearings, and could be without lubricant. It seems over time the grease put in at the time of assembly will stiffen or disappear. If desired you can remove the armature, and remove the seals on the ball bearings. Stuff in some grease, and push the seals back in place.
If you felt any roughness in any of the bearings, be prepared for a failure. Ball bearings don't have much excess room for the balls to pass over bumps in the races. Bumps will lead to more bumps, as a rule.
If the bearings are greaseless, and squeak, it is a matter of time before they start to get 'jerky' and decide they don't want to rotate, which can cause belt squeaks.
If you think the squeak is elsewhere besides the belt, after doing a replacement(which I figure you must have done, that or ran w/o the belt installed), you could spray the belt with some H2O which would sort of lubricate against squeaks for a minute or so, and see if the squeak was helped. If so, you still have something causing it.
tom

i just thought of something kinda neat... and i'd like to hear one of you confirm or not

Don't newer low-power engines turn off the alternator intermittently when ac is on at idle? I recall a later model 4cyl honda or similar being put in the shop because the owner was seeing only 12 volts for the system voltage at idle. he thought the alt was bad. turns out that if you turned off the ac, then the sys voltage jumped up to 13 or 14 volts like normal...
my little baby ranger 2.5l 4cyl at idle says that pulling both the ac and alt is a chore lol

Anyway, maybe that's what's going on. out the truck in gear, and the computer commands the alternator to begin output?

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1999 xlt 2.5l 5-speed Supercab 159k
83 diesel 2.2l na
06 F250 6.0l psd
{click for lol}