Noise - vacuum or exhaust?

[slo95]

I installed a Bassani cat x and mac cat back to stock headers a few weeks ago. I noticed a slight almost kinda "whistle" when accelerating. not at idle, and more so under load. It does get louder as speed increases. Not a chirping or ticking sound though. Just a constant low pitch whistle.
I finished it up last weekend with motorsport headers. Noise is still there.
I lined everything up three times - tighened with a very nice air impact, so much so that I snapped a collector bolt. not fun.
So, does it sound like an exhaust leak? I swear the 400ft lbs at 125psi does not turn the nuts anymore - so they are tight (that and I have reinstalled it 4 times now).

What are the most common vacuum leaks -
it almost sounds like the Idle Air Bypass when it is open too much - like that type of air rushing whistle.