I am not flushed with cash! 

We have run all the water in the unit and in the unit above it continuously, flushed toilets, etc. and have not found a correlation as to when the knocking comes. 

I think I'm going to have to flush out some MFRS that aren't on the MLS.

I saw it flush, and their inspector reported it worked fine in his inspection. 

Visually inspect: Roof, Under sinks, Look at the water heater and hvac for signs of age/wear, look at piping stacks, look for code violations, look at the electrical system, flip light switches, flush toilets, turn sinks on, look at the foundation. 

(one board on deck is warped pretty bad and isn't sitting flush any more, i don't want someone to trip and two, drywall paper around top of shower/tub insert is peeling back from water that slashed up there while showering.  

Example, tenant flushes a diaper in toilet, common sense says don't flush diapers in toilet, if I have to pay 100.00 for snaking a toilet & there is a diaper coming out...you better believe I will try to recoup that 100.00 from the tenant. 

I have already revised my lease a TON, and included an addendum with all kinds of prohibited items from being flushed and has a spot for the tenants to sign that all faucets/drains work and are free of clogs or back ups. 

I've had great water savings on the installation of low flush(1.28 gallons per flush or less) toilets.

Depends on how flush they are usually.