Powerplay

Hi hope someone can help. I recently purchased a 2005 A4 2.0 quattro at auction. (Yeah! Theres my first mistake) The exhaust was very very loud. I then realized it was straight piped but sounded horrible. I purchased a stock exhaust and had a muffler guy install it, leaving the catless downpipe intact. I turned it on and it was as silent as a whisper. But when I got to 60mph on 5th gear I start getting this noise Im assuming is comming from exhaust like a very loud burbble. I downshift to 4th and under 60 mph and it goes away, I accelerate again and again at 60 mph the same thing. Long story short drove all the way home this way. Am I getting too much back pressure for stock exhaust since there is no CAT?
Anyone experience this?