I recently trimmed one of my Black Widow 3 under tabs to about 1/2" inch just to try. This resulted in a significant increase in noise upon release. Makes no sense to me. I shoot 2 under and tried it with 2 different bows with same result. Any ideas? The sound it makes reminds me of a plucked string.

Don't know what it could be - mine are all trimmed short.  Are you using a deep hook or is the string on your finger tips?  A finger tip hold might result in a 'plucking' sound I would imagine.

HHhhhmmmm not sure but it sounds like you might be plucking the string.
Try a longer tab and see if the noise goes away  

Deep hook, noise does go away with longer tab.

I switched to a blackwidow tab 4 moths ago. After 10 weeks my hands were so weak I could barely shoot. I personally think those tabs are to thick and feel like a brick.

I would spend the money and get am AAE cavalier tab and focus on forgetting the pluck. You concentrate on that for much longer and I guarantee you'll start dropping your form to see arrow flight.

I could be way wrong