Some free YouTube Music users are starting to see new banner ads between the cover art and the Now Playing controls.
YouTube Music Premium costs $10.99 per month, while free users are exposed to ads after every couple of songs on the Now Playing screen.
The ads on the free version of YouTube Music include a 'Sponsored' label, a counter for the number of ads, a timer, and cause the progress bar to turn yellow with greyed out Now Playing controls.