CD43/Alpine radio oddity

[pgrindstaff]

My fairly recent purchase of a CD43 radio has been good to say the least. It is better than the stock units that were in both of my cars and has given me no trouble until recently.

When I bought my 540i and decided to sell my 530i I swapped the CD43 with the alpine unit that came in my 540. Shortly after that I have been experiencing a strange tapping or ticking sound on all frequencies on AM/FM/CD. This noise, while hard to accurately describe, is a fast paced tapping sound that does not change in relation to other factors such as RPM. This noise was not present in my 530.

As a test I took both radios out of both cars and retested the CD43 in the 530. No tapping/ticking noise. When plugging the alpine unit back in the 540 there is no noise, nor has there been since I have had the car.

This issue is very perplexing to me as it is not consistent between cars and is obviously not a problem with my radio, which can be deduced from its normal behavior in my 530. Does anyone have any idea what my problem could be and what I should check? I am sure all connections on the radio end are tight. Any ideas are welcome and I apologize for a long boring post.

edit: The engine was off for all testing last night, key was turned to ACC

Thanks,
-Patrick