What could this Isuzu problem be?

It might be the pressure relief valve on the a/c compressor. If it happens more than a few times, you'll begin to notice a loss of cooling from the a/c. This can be caused by a cooling fan that is not working at idle or very slow speeds or it can be caused by a restriction in the a/c system. I'd take it to an a/c shop and have it inspected. If tit is he pressure valve relieving, there should be an oily area behind it.