USPO postmen blowing a whistle at each stop?

Yes, I’ve been watching too many old cartoons on-line…

Googling “postman’s whistle” bringing up lots of old song lyrics, along with real artifacts of such a practice (i.e. old postman whistles for sale),
So, did USPO (pre USPS - probably pre-Korean war even) mailmen, upon delivering mail to a residence, blow a whistle (to alert the residents, I guess)?
Was the whistle blown for regular deliveries, or only for important letter or package deliveries?
Was this practice widespread on most mail routes, or was this practice rather rare and simply became entertainment industry shorthand for an important letter arriving?
Did they “postal-whistle” deliveries at businesses too (say small businesses like a small store)?
If the practice was common, during which eras was it common, and when did it fade away?
Did this practice apply to RFD routes (mailbox at the side of the road style - put the flag up and blow a whistle)?

Extra Bonus questions - as I understand it, in the USPO era (a short span from1792 to 1971 or so) some urban areas had multiple daily deliveries - what was the criteria for this, did residences on the route get the same delivery frequency, and when did this practice fade out of existance?

on a roadside box the red flag is up if a letter is waiting to be picked up. the carrier wouldn’t want to put it up because it might be up the next day with no outgoing letter. the homeowner expects it down when the letter is picked up. other uses are confusing.

dunno about the whistle. it would seem to be a problem in a city because any person could hear the whistle for their neighbors too.

Way back, before WWI, the average household received no where as much mail as we do today. Url below contains info from the Smithsonian.