I have a 1999 S10 Blazer 4x4 and this summer the compressor has developed a knock that is louder at times then other times. It still blows cold while driving, but only blows warm at idle. The compressor clutch never shuts off so I know it isn't the low-pressure switch or low coolant. What could be the possible causes and what should I look at first? I'm assuming something might be partially plugged since the compressor still works? Or is this what a compressor does before it implodes?