Like bits are going though it.. clicky clicky.. But not all the time... they will be like pauses between for about 10 seconds.. But there is nothing in the water.. so I don't thinks it taht.
My pumps do that some times.. maye you got a few bubbles still in the loop as every so often.. say... once per 4 hours or so, my pumps make a swooshing sound for 2seconds
or depedning on which rad you are using, take it out of the case... and unscrew the screw which is fitted on the bottom end opposite side of the connectors (facing up of course), to let any bubbles trapped free.