Noise from new Sunrace cassette

After installing a new Sunrace 40-11T cassette (minus one cog), a new
chain and a new rear derailer the road bike can now climb hills much
better. 40T as biggest cog versus 32T before. Woohoo!

However, on the middle and three larger cogs (it's now a 7-speed) there
is a distinct vrrrt .. vrrrt sound when under heavy load. Maybe from the
chain because with a derailer setup it'll never run 100% straight. Hard
to say. The noise appears briefly twice per pedal crank rotation and
always on the power strokes.

The chain is a KMC Z50 that should be suited for 7-speed and I looked,
it doesn't rub against a neighbor cog. Maybe a "teeth exit grinding"? On
the 3rd cog from the largest it's really weird because that has an
almost perfect chain line when on the small chain ring up front.

Anyone heard that before? Can it simply be ignored? Or maybe it'll go
away over time?

--
Regards, Joerg

http://www.analogconsultants.com/