F150 ac compressor?

One of TWO possible things! Your AC may need a RECHARGE as they act this way when the freon is too low! There is a HIGH and LOW pressure sensors that need to be stateside.  another THING is that your IDLE AIR CONTROL VALVE and THROTTLE BODY may be full of SLUDGE! This will happen if you are using very el cheapo oil or VALVOLINE which is an ASH and WAX based oil! It is NO GOOD and makes harmful ACIDIC vapors that et up rubber parts ( PCV HOSES) and SEALS and GASKETS! >>>>>get a can of AMSOIL POWER FOAM and follow the AIR INTAKE SNORKEL to the THROTTLE BODY. Remove this snorkel and CLEAN , CLEAN,  CLEAN the throttle body! ON FORD cars the IACV is held in place with TWO 8mm screws near the T body! REMOVE THE screws, and clean the IACV by HAND! AMSOIL P FOAM s dissolves sludge and gtar on O CONTACT! i even use a mixture of AMSOIL with GUNK ORANGE ORIGNALengie cleaner to put directly into the crankcase through the OIL FiLL CAP! Then I drive for about an hour GENTLY but FIRMLY As the solvents cleanup and MOP UP the entire ENGINES INSIDES. >>>THEN USE ONLY PENNZOI LULTRA PLATINUIM or ULTRA EURO that protects down to 35 below ZERO and gives you an AMAZING extended warranty out to 500,000 MIELS! it is MADE from natural gas and CLEANS as you DRIVE!  >>Once you clean the T body and the IACV< your engien will get a perfect idle and GAS MILEAGE will go UP as you drive in and out of traffic and in STOP and GO traffic! Teh AIR FUEL mixtuer wil return to SHOWROOM and your PISTON RINGS and SKIRTS wil get CLEANED of all the baked in MUCK made by cheaper less refiend OIL! GOODLUCK!  

Compressors rarely fail.

Fords are however famous for bearings that make noise.

It may be coming from a idler pulley or from the belt tensioner.

When the compressor kicks in it puts added tension on all the bearings along the path of the serpentine drive belt and those bearings are cheap and poorly lubricated sealed bearings that are designed to fail.

As always, look at the least expensive suspects first.

My 2006 F150 with only 44,000 miles had noise coming from every bearing under the hood at some level

I had to replace the Tensioner because that bearing was shot and causing the belt to make noise and wear.

After that the noise was less but not bone.

I then found that the sealed idler pulley bearing was making noise so i injected grease into it and again the noise was less but not gone.

I then found that a sealed alternator bearing was was making noise so a injected grease into IT.

And finally after that the engine was as quiet as it was when it was new.

I found myself rolling down the window when stopped and straining to hear the engine running.

I later found out the cheap Chinese bearings Ford used left the factory with very little lubrication which is what was causing all those sealed bearings to fail early.

To check the AC clutch for bad bearings take the drive belt off and spin the outer portion of the 

ac clutch to check for noise and try moving it in and out to check for play. When the AC clutch engages the PCM increases the motor rpms to handle the extra load at idle so the motor doesn't stall.

Remove the serpentine belt and turn the compressor clutch, the compressor pulley, the alternator pulley, the tensioner and all the idlers by hand and check for any binding.  If the compressor or alternator  checks bad, get (a known good) one from a junkyard or an online recycler company. 

But really, there is likely nothing wrong with your truck. 600 rpm with AC off and increasing to 700 rpm with AC on sounds normal.  The compressor turning off and on is normal as it is just cycling with the AC on.  You can check the rpms with an AC manifold gauge hooked up.  You can also check desired and actual rpms with a scanner with AC on and off.  Good luck.