I have noticed that after I have left my X1C(ams) idle for long period (8+ hrs) when I start a job it emits a high pitched noise (almost like a whistle). Does this for about 20 secs then stops. Print comes out fine. If I am running it all day with different models never get this. No errors are displayed.
Thats not a whistle its not homed correctly
Ok Thanks , New to Bambu. I will look how to home.
I can’t find any information about how to “home correctly”, is there some other term I should search for ?
I have an opposing viewpoint to William’s 
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The noise the printer makes when homing, when it runs in to the travel limit but doesn’t realize it, is something I’d describe more as a “grinding” or “growling” than a whistle.
I can’t think of any reason why the printer might whistle when you start it up, with one exception - fans.
I have had plenty of small fans go bad over the years, and sometimes when they do they’re noisy at startup, the bearing chatters a little until some heat gets in to loosen things up. Sometimes, tapping the fan lightly can “snap it out” of the resonance and the bearing quiets down for a while.
That chatter is something I might describe as a whistle sound, depending on how fast the fan is turning.
Printer shuts down fans after entering idle. Them starting up again may be the source of the noise.
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OK the “whistle” is more of a high pitch sound like a smoke alarm makes. I will check to see if coming from fan.
I was able to catch this again and record it ( but forum will not allow MP3 upload.) , it happen after I swapped in a new filament roll in slot1 (a1) on the AMS . I don’t know why swapping in filament would hve anything to do with this but it’s the only thing different. After it does this printer is working fine.