If your noise happens when you slelect first gear at a stop, I'm going to guess the high idle issue is causing your gears to grind when you engage them. The shafts in the transmission need to be moving similar speeds to enage smoothly. If your input shaft is spinning 3000 rpm and your output shaft is sitting still, they won't engage smoothly.
Rarely is the question stupid, but sometimes the answer is
you need to run everything you read online through your own personal BS meter to determine if it makes sense to you.