Toto washlet s300 has developed slow leak from behind brass connection and plastic.

Start cleaning flapper. If it does not work then replace toilet flapper.

check You Tube https://www.youtube.com/results?search_query=toto+washlet+repair
and see if one of their videos will help.

That whining sound is most likely the small fan Toto has to "cool" the electronics. Very similar to a laptop PC fan. The fan will eventually go off, ~ 20 minutes. If you would like it to go off immediately, simply push the power button on the seat - off, and then again for on. Do this action while you are not sitting on the seat, otherwise the cycle will start again. :)

The supply line running from the junction valve to the seat is kinked or pluged in some other way way. take that line lose at the juction valve and see if you can blow air through it.

Clean the internal water filter. See page 24 of the instruction manual. If you don't have the instruction manual you can download it.

two screw if you look in frent botim of tank that you flush normally plastic open and the seat will come off and you put back the new one the same way back

Poor some clr in the washer bottleand block the nozzles as you run the clr through. they will clear up, but don't leave the clr on your paint or in the washer bottle. Rinse everything out and off.

I had this problem and here is the solution. Remove the cover where the water supply hose goes into the diverter fitting. It is on the left side of the toilet and has a single screw. It is where the washlet shutoff valve is. On top of this valve (diverter) are two brass fittings. One is built into the diverter and the other is coming from the top. The top fitting should fit into the bottom fitting. If you look closely, the top brass fitting is connected to a gray hose. The two brass fittings need to be tightly coupled. If they come apart (by more than 1/8 inch) water will not enter the washlet. There is a check valve that will prevent water flow. Depending upon the manufacturing dates, there are units with and without fastener guides that hold these two brass parts together. Mine had a plastic piece which did a poor job of holding these pieces together. Customer support told me that I needed a metal fastener and instructed me to remove the plastic fastener. When I held the two brass parts together the washlet worked fine. So, when I get the metal fastener I expect the washlet to work again.

from where?? If it is leaking on to the floor from the base of the toilet you need a new wax ring installed,,, if the water is comming out from where the tank bolts to the bowl you need to get the proper kit since it;s a toto they are one of a kind and don't have any universal kits that will work, call or e-mail toto tell them you need the tank to bowl kit, might as well tell them to send the flush valve also and just do the whole thing good luck