The strange thing is that it wasn't a problem before but it is now. It could be that the amp is defective now...
Do you know anyone with a spare aftermarket amp that you can swap in to test if the hiss is still there? If it is, then it is a wiring or source unit problem. If not, it's probably the amp. The PDX series are still D-Class amps which are generally noisier.
2007 BMW 335xi 6MT TiAg