It's on the tip of my tongue and stuck in my head...
 

So it's a song that sounds very similar to Lovely Day by Bill Withers. But I believe the song has a very similar bass line and uses the phrase "diamonds in the dark" or "diamonds in the rough" as one of the chorus lines and then breaks into a whistle that sounds like the bass line... I think the melody has a hard keyed piano...

Still driving me cuckoo!
 

Ok, so I was playing with a piano app and if you tap this n a keyboard, I think I picked the melody out.

D-ddd-c-b-g then

D-ddd-c-b-c-b-g -- g#-a-g#-g-f#-g