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AC Air Conditioning gets loud - whirring?

Strange thing - after the AC runs for a bit, there is loud revving that goes up and down with my RPMs, and lags a second or thereabouts behind the RPMs.

In other words, it’s a hot day. I get in the truck, start it up, and turn on the AC. I start down the road, nothing unusual. But after five minutes or so, the engine noise is matched with another sound as I accelerate. The higher the engine RPM, the higher the additional whirring noise. The engine RPMs go down, so does the whirring.

Again, the whirring lags behind the RPMs by a second or so. I accelerate and, then after a slight lag, the whirring begins/increases.

The AC has been adequate but never been strong, even when the truck and AC were new.

This whirring has been going on for a year or so. I’ve had other fish to fry, but some of the other problems have been addressed and I thought I’d look into this one.

This whirring reminds me of a turbocharger, both in sound and how it rises and falls with the engine RPMs. Any ideas?

I’m tempted to call the sound “roaring.” I guess it’s more roaring than whirring.

In any case, the best my feeble imagination can come up with is the compression builds in the AC system, and after a while that pressure causes something that’s rotating to make this noise.

Does turning off the A/C fix the noise? Most likely it is a belt or belt tensioner bearing.

Turning the AC off - the noise does go away after a few moments, not instantaneously. Belts are new, look good, tension seems good.

I’ve heard of an issue with some Toyota orifice valves making noise. Perhaps that is your problem?

I am wondering if this might be the fan clutch engaging. IIRC the coupling uses a viscous fluid that would cause an acceleration lag. When the air temperature exiting the condensor/radiator passes a certain level, the fan clutch will engage and make a lot more noise than at a cooler temperature. If the fan clutch is engaging, you might notice the noise that starts with engine startup but goes away soon thereafter until the engine is warmed up again.

If that is what the noise is, it is perfectly normal.

orifice valves? Are they part of the AC?

Yes, just before the evaporator.