As clean as a whistle
Definition 1 of 2
Pro Tip 1/3Metaphorical Use
The phrase is a metaphor, comparing something's cleanliness to a whistle's clear sound, not its physical state. 
He scrubbed the car until it was as clean as a whistle.
Pro Tip 2/3Exaggeration for Effect
Often used to exaggerate how clean something is, implying it's exceptionally clean. 
She left the bathroom as clean as a whistle; it was almost sparkling.
Pro Tip 3/3Common Collocations
Commonly found with words like 'spotless' and 'sparkle' to emphasize cleanliness. 
After the deep clean, the floors were spotless as a whistle.