Exhaust whistle

[MacGyver]

Got a 2000 D22 double cab, TD27 diesel, 5 Spd manual That I have fitted a factory turbo to.
Been going great. Heaps more grunt etc.
It’s just started to whistle out the end of the exhaust pipe. Standard system at the moment.
What’s causing this?
The exhaust system is the original and getting a bit tired.
I’m wondering if the turbo and related hotter exhaust has blown out some baffles or something?
I have some 2 3/4” stainless pipe and bends to replace it with in the near future but for now I just want it to stop whistling.
It’s not really loud and only at lower revs.
Still annoying though.