tl;dr: I also see a problem, but be patient. It may take 60 to 120 seconds to get HD.
I'm on TalkTalk Business (resold via Pulse8), ADSL2+ synced at ~18/1. My traffic is supposedly prioritised over TalkTalk Retail traffic. I've used Amazon Video over that connection for 18+ months. I've recently noticed it's taking far
longer for the HD stream to kick in.
When I was watching The Man in the High Castle
a year ago, the stream would start with a fairly decent SD picture, switching to HD before the end of the opening titles. I don't recall any problem with lipsync.
However, each of the three Grand Tour
streams I've watched have started fairly OK, then actually down-shifted to a horrible, low-bitrate SD stream, before finally managing an HD picture after about 90 seconds. I'm also seeing lipsync issues, but not hearing the noise you describe.
Is that what you're seeing? Or does it never get better, even if you wait a few minutes?
3 km line on THTG: 18/1 Mb/s with Pulse8