oyskin

So I have a mk1 with 1.8t and 280k km. It's chipped with a fake forge recirc valve. Lately I have had this woosh sound when giving it throttle, and not the same power as it used to. I had this problem for months before the engine light came on, I scanned it and it said something about too lean mix. Any suggestions to where I should check first, and how? I'm thinking vacuum or turbo leak somewhere, what's the most usual? The turbo has never been changed, so I was expecting it to be done soon anyways, but the turbo itself seems to be working okay.