Last summer i converted my R12 ac to the r134a sytem with the DIY conversion kit......My question is about checking the oil level in the compressor.....How to do it??? when i did the conversion the r134a said with lubricant oil on the can.....The problem is the AC is working but has a rumble when it is running mostly around idle speed.... With the clutch disengaged there is no noise.......any advise ??? Thanks