My kitchen sink has been not working properly since last Monday anyone can suggest me good or reasonable plumber?

If it is hot in the kitchen, then once the water has reached the shower it should be hot. The leads me to believe that the issue is with your shower mixer unit. Nearly all are thermostatically controlled.

Or, if the heater is downstairs and the bathroom is upstairs, you don't have enough hot water pressure. You need a good plumber with a pressure tester (called a manometer).

Here is the site for the manual. You should be able to find it in there.
http://www.palomatankless.com/owners/homeowner+use+care+manuals.html
As for the noise, some models did have recall on them for defects.

Check for crossover.
Bad check valve.
http://waterheatertimer.org/Crossover.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Very simple - call a Plumber. He will fix your plumbing problem, and any foreseen potential problems related to your water leak. He may also suggest replacing your 6 year old water heater. Of course ..... for a professional cost.
However, depending on your time, skills, proper tools, patience and luck, it's easy enough to perform yourself:
1. Switch off the water heater Breakers (220v) in your service panel.
2. Turn off the water supply to the water heater.
3. Attach a water hose to the drain faucet. ( a good time to flush sediment -calcium deposits)
4. Open a hot water faucet to vent (above the level of the water heater drain - kitchen sink)
5. Remove the suspected leak components. (a union connection is a good place to start)
6. Install New pipes -3 wrap all male threaded ends with teflon Tap. Tighten securely - using 2 wrenches.
NOTE: Before refilling - you can turn on the water heater supply valve for a few minutes, to flush out any lingering calcium debris.
7. Shut off drain valve - remove hose. Shut off venting hot water faucet.
8. Turn on water supply valve to fill water heater.
9. Open a hot water faucet to relieve air pressure while filling - preferably the bath tub faucet, it doesn't have a screen that might clog.
10. When water flows without air - shut off tub faucet.
11. Look for leaks.
12. Switch the water heater Breakers on.
Note: 50 gallons should take an hour or more to heat up. Turn on a hot water valve momentarily within this heat up time to relieve thermal expansion. ( the PT valve on your water heater will perform this task if pressures/temperature exceed new water heater tolerances - it's just a precaution to introduce these fluctuating pressures on your fragile 6 year old unit)

These are just basic instructions - assuming you fill in the common sense and work safe steps.
Good Luck - The Skill will only come after trying.

Turn Hot Temperature down lowerthan current setting. Make sure that Heater operates properly before using. A defective temp sensor can SCALD and BURN. What TYPE of Terrible SOUND? Do you have anyone tht is the least bit mechanical to observe and direct you to the right source. IF your husband enjoys cold showers this sounds like an opportunity to suggest that with WARM baths the two of you can have some Peaceful time! And both come out clean!

is there a hose connected to a faucet with hot and cold open causing h&c to mix

is this a single handle faucet? if so your cartridge may be clogged.If not your supply line is clogged,check the valve also.

Will the water stay hot in the kitchen sink? Try to isolate the problem. If it is happening at all hot water locations, then it will be related to your water heater. If just in the shower then it is related to the mix of water ratio coming out of the shower hear. If its only the shower, then you are getting too much cold and not enough hot through the pipes. This can be very simple to fix by replacing the handle going to the shower. If this does not work then you may have a problem in the hot water pipes. After you isolate the problem, please post another question here.