How to repair Grohe kitchen faucet pull out sprayer

Most likely the humming is due to a defective or damaged check valve on your water METER and only translates the sound through your extrol expansion tank. The check valve on your meter should be replaced. The noise stops when you run water because the faster flow through the check valve opens it completely. Most likely you have a running toilet or dripping faucet that let's water seep through the check valve causing it to partially open and creating vibration in the bad gaskets on the check valve.
#1 have the check valve replaced
#2 find and repair the water leak

1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

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low pressure

There is a minimum water flow required to operate your Bosch tank-less water heater. It is around .75 gallons a minute. There are three things you can do to help the flow.
1.Turning the temperature down will help increase the flow.
2. Installing higher flow aerators on your faucets.
3. Clean the Bosch tank less water heater.
You are not alone in having this issue. The most common problems are low flow faucet aerators and low flow shower heads.

The thermostatic stop is set too low. Havent set a grohe before but most other thermostatic shower valves work similar. pull off the handle(however a grohe handle comes off . . ) there should be a notched ring or stop which keeps the handle from going all the way hot. pull out the ring and turn it a few teeth hotter put back and reinstall handle try water temp and adjust again if necessary.

Do you mean the other faucets in the house are hot and only the shower is warm? If so there is an anti-scald adjustment on the faucet that needs to be turned up a few notches. Havent adjusted a Grohe faucet but most faucets you rmove the handle and behind the handle is a nothced adjustment ring that prevents the faucet from getting too hot. This ring, or stop, is pulled out and twisted clockwise(or counter clockwise to set higher or lower max temp, then pushed back. maybe you can look up grohe to see which way ring or stop adjusts for hotter water.

this is a problem they didn;t tell any one about, there is a min of 1 gpm that you have to run for it to sence for the unit to come on, if not the unit will not detect the water is even runnig there for will not come on,, as for the kit faucet, it might have a temp control and it may need to have a cartridge change, the water comming from the tankless is nothing like the old tank heater, the water only losses aprox 1 or 2 degree where a tank heater losses almost 5 or more because of the way the cold water is fed into the unit,, thats whay the sprayer works and the faucet doesn't ,, you have control on the sprayer unlike turning the faucet on,,

call grohe and have them send you the hole unit hose and all they warranty it it for life it is obvious there is a slite crack in the head base, they will send it for free

grohe is a faucet all by itself ,,, I would recomend to have you call there repair dept. and explain this to them, you said you put in the new cartridge and thats about all there is to it, when you put on the handle make sure it is not over tightened, sometimes it pulls on the cartridge and makes it come on by itself good luck

I found a way to get warm water in the shower, but I'm not sure it qualifies as a solution. After trying everything else I thought that maybe when I was feathering the faucet in the shower toward the cold side so I would get warm water that at a certain point it blocked the flow on the hot water side enough so that the sensor in the hot water heater sensed there was no demand in the line and it shut the water heater off. To test this, I went to the kitchen and turned the kitchen sink faucet all the way to the hot side and then turned it on so that I got a small flow of water. I waited for the water to get hot to be sure the hot water heater was working. Once it was hot, I left it running to keep demand in the line and then I went to the shower and turned the faucet on. When the water got hot I started feathering the control towards the cold side and soon I had warm water. So I think I'm right in that the sensor in the hot water heater was shutting the water heater off when the demand in the shower was lowered to a certain point when I was trying to adjust the water temp in the shower.

So now I can get warm water, but it means I have to leave the hot water running at another fixture in the house to create a false demand in the system so the hot water heater doesn't shut down. Is there a way to adjust the senor in the hot water heater so that it will stay on when I'm using the shower only, so I don't have to waste water by running another faucet when I want to take a shower?