Seems like you've already got this one sorted by replacing the downpipe, but I just wanted to add my experience.

My APR downpipe also has a bit of resonance - slightly noticable around 2250rpm then a bit louder about 5500rpm. I mentioned this to Guy shortly after it was installed just as an observation - I don't really notice it any more so it's not an annoyance but it is just a different characteristic from the stock parts.

There is certainly an increase in the induction noise you can hear (especially with the windows down) - so whistles, pops etc from the turbo are far more obvious and took me some time to get used to (ie it took some time to realise they were 'normal' noises not signs of something wrong).

The downpipe did come loose from a mount shortly after it was installed, causing an exhaust leak and the change in note was instantly noticeable - obviously it sounds terrible with the leak. Thankfully it was a quick (and no charge) fix and it's back sounding the way it should.