tap the constant line. The switched will turn the radio on and off. Is the noise high pitched? Is the popping sorta quick? There are many possibilities here. I think you said it happens while playing a CD as well as while listening to the radio so it's not your spark plug wires. Could be your speakers. If you're sending too much juice to the speakers the amplifier (in the head unit in this case)sends DC voltage creating an annoying high pitched popping sound. My deck is doing this right now but I have an external amp and just need to turn the gain down a bunch. It's harder on your car. Is there a local stereo shop near you? If so take it in and have them give a listen. Be chit chatty and friendly, they love trying to sell you stuff if you're just looking for advice. I think that might be a good way to go. Try to avoid the big chains it seems to be hit or miss with the ones I've been in. There are car audio only chains like car toys that are very knowledgable and willing to help but talk to the local guy too. I think anybody with some car audio experience who gives a listen to your stereo will quickly be able to diagnose your problem and advise you how to fix it. Keep us posted.
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Portland, OR 1987 240 dl wagon 'rustbucket'