There could be a couple of answers here. It depends on how old your water heater is...if your heater is relatively new (say under 5 years or so) it could be a bad heating element or perhaps the temperature needs to be adjusted. If your heater is older (8-10 years or more) then the most likely problem is a build-up of calcium and minerals inside the tank. This build-up can significantly reduce your water capacity over the years. When this happens, the only answer is to put a new water heater in your home. Hard water can reduce the life of your appliances (washer, water heater, dishwasher, coffee maker, etc.) by HALF!
With most disposals, there is a reset button or switch located on the bottom. Sometimes, if they jam up due to an overload of debris, they will automatically trip the switch so the unit won't burn up. Press the switch at the bottom to reset. If there is a clog in the sink - clear that manually. If the reset does not do the trick, it's possible a replacement might be needed. Most disposals and drains do not handle coffee grounds, potatoes or carrot peels and the like very well. You are better off throwing those scraps on the compost pile or in the garbage.
Believe it or not, it could be caused by one or more of your toilets. The next time you hear it, look inside your toilet tank and then put your fingers on the rod that the ball is attached to; if the sound stops then you need to have your fill valve changed.