Wiktionary
clean as a whistle
a. 1 (context simile English) Very clean 2 (context simile English) Completely innocent; beyond moral reproach.
Usage examples of "clean as a whistle".
I've tried all kinds of things to reproduce it, but the whole machine is as clean as a whistle.
His gun and ammunition, for example, could even now be tipped into a lake in Scotland, so that he can present himself to his own police on his return as clean as a whistle.
Everything clean as a whistle, and we've even collected a piece of lettuce.