So I installed my DP with a flat flange. It doesn't mate well with the stock doughnut flange. Right now I just have a doughnut gasket in place and wrenched the hell out of the two bolts.

Fired the car up and I hear whistling.

I think I might have a leak coming from there.

Any ideas on how to make this a better connection?

first turbo car, first dp mod.

 

that might just be how it sounds
put some soapy water on it and see if you can detect the leak
when i put on my titek DP, it sounded like a leak but it wasn't
just the sound of the new DP
good luck

 

So you mated up a 3 inch dp with a 2.5 inch catback with no adapter? If you did that then you will definatly have a leak.
If you want to make the connection better then you should get a 3 inch cat back to mate up with a 3 inch dp.

 

u will hear a turbo whistle,..