Acceleration whistle: '03 Maxima

Hi. I have a 2003 Nissan Maxima, 90k miles. I'm getting a very faint whistle or whine sound when I accelerate. However, it's only happening when I depress the gas pedal in the first half an inch or so (when I press the gas pedal further than that, the whistle/whine stops). It doesn't make the sound during idle and I can't replicate the sound in neutral, only when the car is moving. And speed doesn't seem to factor in as the whistle occurs at different speeds. It really seems to be tied to the first 1/2 inch depression of the gas pedal. I just had a new muffler system put on (from after the Cat back to the tail pipe). I'm fairly certain the noise wasn't there before (though I've brought it back to the shop and they say it's not related). Again, it's a very faint whistle, but it's audible inside the car and quite annoying. Any help would be greatly appreciated.

...certain the noise wasn't there before...

U said it: New exhaust system ... has some leftover weld stuff from factory -or- a leak. Complain to the shop that installed it :( or drive always WOT.