Had a Zoeller Model 98 sump pump with a Flotec checkvalve. All was
good and the pump operated quitely.

This weekend, I added a Basement Watchdog Big Dog battery backup pump
to supplement my Zoeller. I wye'd off the 1.5" main line and added
two new flapper type checkvalves from Lowes (each one about 12" above
the pump). I discarded the old Flotec flapper type checkvalve.

The system seems to work fine. Only difference is that now when the
Zoeller cycles off, I hear a mild clunk that I never heard before.
Any idea how I might quiet it down?

BTW, there are 1/8" downward angled weep holes just below each of the
checkvalves to prevent air lock.

Thanks,
Kevin