Screaming Compressor - High internal pressure? Approx repair costs?

[sosolid4u09]

Hi,

My compressor is making an incredibly loud screaming noise. I read online this can be due to Dangerously high internal pressure.

1. Could it be something else? i.e. would a leak cause this noise?
2. Is there a way I can diagnose what the problem is myself?
3. If it is excessive pressure, what is the rough cost to repair? Ballpark figure?

Thanks

Update:: Interestingly, i ran it for several hours the last few days and there was no noise whatsoever. Worked perfectly. Then just now i tried it and its screaming again