Belt Tensioner or Air Compressor

2003 GT 4.6 about 90,000 miles

I've got a strange problem that I want to get your opinion about. I've searched for the last hour and can find somewhat related issues, but not exactly what I've got.

The car runs fine with no sounds or issues at all. Then you turn the air on. Under the hood, the ac clutch pulley starts to spin immediately with no added noises (well, the pulley spins all the time, but when the air comes on the face of the pulley, or the compressor itself starts to spin). Wait about 15 seconds after the air comes on and the belt tensioner jumps about a half inch, then a horrible and loud screech/rattle/whine starts. Noise remains as long as the air is on while it is making noise, the belt tensioner doesn't move. Turn the air off and the noise continues again for about 15 seconds, then the tensioner jumps a little again and the noise goes away.

My first thought when I heard the noise was that it was the ac clutch pulley or the compressor. After watching from under the hood, it seems to me that the ac clutch pulley and compressor are fine, but that something may be wrong with the belt tensioner.

What's ya'lls opinion? What should be my first step? Replace the belt tensioner and belt since they are relatively cheap and rule that out. If its still there, try something with the ac?

Thanks in advance for any help.

If it matters, one of the idler pulleys was slightly, very slightly loose. Cheap enough to replace, should I do so, or does slight movement not really matter? With the belt off, and turned by hand, it made a slight rattle noise.

Again, thanks in advance for any helpful replies. I haven't done much work to the mustang and am still a novice with it. It's my wife's car. I dearly love driving it, but just haven't done much work on it, so I'm relying on you experts. Thanks.

Thanks for the reply. I have no desire to change the tensioner if that's not the problem. So if it turns out to be the AC compressor shaft seals, how is that fixed? New compressor, or can it be fixed?

I didn't try any belt dressing, but I did take the belt off and spin the compressor pulley. It seemed fine to me.