Hey everyone. This may sound like a wild goose chase, but here it goes. When I bring the engine up to 2600 rpm, I hear a weird noise coming from the engine, almost sounding like grinding (this is with the A/C off). I was revving it to check out my A/C problem: the airflow slows down when the engine is revved above 2000-2500. What is so special about that RPM? I was driving on the highway before and I noticed that the A/C was on high, but the airflow slowed down when I revved it to get up hills or pass. I did the rev with and without the A/C on with the car stopped and it has the same thing. Now here are some important facts that I gathered that are important to determining the problem

-ALL fluids are new, clean and full (including the engine oil)
-This was after driving 40 miles at high speeds today
-Good oil pressure (30-40 at idle, 50-60 while driving) with Mobil1 5W-30 and Rislone
-Temperature never goes above 210 even on hot days, normally 1-2 notches below that and it was 85 degrees today.
-Freon is full and there is plenty of oil in it (I normally throw a can of oil in there when I charge it for the season and the compressor is quiet. It has a small leak somewhere).
-Car has a Posi-Lok system and the actuator still works and holds vacuum when the car is in 4WD
-No known vacuum leaks: Brake booster holds vacuum for at least 2-3 days.
-Belt, idler pulley and tensioner were just replaced, but belt makes squealing noise if I start it and it is wet out, but I cannot find any problems with it. All pulleys rotate freely and without problems
-Water pump rotates freely and has no bearing play
-I do get lifter tick on startup, especially when it is cold and I do let it warm up. This has been doing it since 58k when I got the car. I am at 102k now.

Other than the noise that I mentioned, the car drives fine and I cannot find any problems with it and I cannot hear anything when it is moving (in fact with the windows closed, I could hear a whisper when driving at 40mph). Any ideas?

 

Forgot to mention: Distributor cap, rotor, wires and plugs were replaced at 99k. All parts are AC Delco except for the wires, which are Bosch. The car also has a CAI and I just put a new filter on it at the same time. PCV valve is new, but makes noise at idle, but it goes away once I rev it above idle. Not sure what any of that has to do with this, but I just wanted to mention this before anyone says "When was the last time you did a tune up?".

Also, when this happens, the speedometer needle starts to jump to around 5-15mph even when the car is not moving when I rev the engine. I have no idea why and it is accurate when the car is moving.

 

Did a repeat test today after driving it for only 3 miles (engine was warm, temp was around 65 with some rain and oil pressure was 40-45 at idle) and it didn't do it. Smooth and no weird noises A/C on or off even close to 2800. Maybe this is an oil issue relating to temperature? Maybe it is the lifters not getting enough oil because of the temperature? Maybe Mobil1 HM 5W-30/Rislone can't handle 85 degree temperatures and long trips because the pressure drops about 10psi? The pressure was only 30 at idle after driving a long distance. Maybe I should use something else next oil change? If I can find it, I was looking at Valvoline MAXLIFE FULL synthetic.