Charles, I have the EXACT SAME ISSUE! HOW ODD..
In Source position it's louder than in tape, you can hear the ringing of the flywheel or it's teeth as it rotates. I can hear it in 7 1/2 or 3 3/4 ips, but not as high pitched, perhaps 1/2 the pitch. Like you I see no ground or other issues, all...