I’ve had a wastegate rattle problem ever since I got the car
it is rattling even when it’s all warmed up
it tends to happen when the gear is about to change
it’s very metallic

anyone with this issue
I live at high altitude
(denver) maybe it has something to do with that

 

A little wastegate rattle is normal. Mini even issued a TSA about it a few years ago. They rattle because at rest they are open, not closed like most turbos. The actuator pulls it shut using vacuum.
What you are describing almost sounds like a heat shield or something like that rattling at a certain rpm. Check the heat shield over the turbo and around the downpipe.