Just sounds like partial blockage to me have you tried the basic plunger to see if that does anything to the problem. Is the water draining as normal or slowly? i had a what sounds like the same sound you are hearing i just got the gd old plunger out and gave that a good go and was fine for a while but then that sound came back again i kept plunging for a couple of days and its been 2 weeks and its been fine if that doesn’t do the trick get some Mr. muscle
