So I modified the stock muffler by drilling a hole into internal barrier.

The car sounds great at idle and speed-with the windows up.

But with the windows down I hear a "flutter" or a hissing from exhaust.

What am I hearing? How do I get rid of it?

 

How big of a hole did you drill?
I did and inch and an eighth and don't hear anything odd.
Is it possible you're hearing your turbos?

 

If it sounds like the video when he revs it,


then it's your turbos.