Baer, you're right these engines are very noisy! I forgot to mention that I did use a 2ft piece of hard rubber hose to listen for the source. That's the only reason I was able to narrow it down to the driver's side about midway down the engine (maybe lower). From there, the acoustics of the engine compartment and other engine noises made it difficult to get more accurate.
I tried listening from under the car too but couldn't get the hose close enough to the header-cat junction to get a definitive answer. I also melted part of the damn hose in the process and stunk up the garage
SouthArk, I agree. It really seems like a leak. Some folks have had problems with these Berk graphite gaskets, so I'm just going to pull the cats this weekend. Will post pics if anything looks out of the ordinary.
Side note: I'm placing an order for ART pipes and copper RTV!