We are manufacture of diesel enigne and diesel generator

The reduced atmospheric pressure at high altitude means the standard carburettor settings will not allow sufficient fuel to flow for efficient running.
The dealer or manufacturer should be able to provide details of alternative settings - the jet sizes are usually increased a few points for every 5000 ft above sea level.
The cooler temperatures experienced at high altitude would probably change the engine oil specification to a lighter or thinner grade.

diesel oil 20w -30 for normal ambient temp areas

Check your oil. If you don't have good oil pressure it will shut down.

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Increase the speed for constand power output, or check the load it maybe more than the genset can power.

Caution
the manufacture is recommending a cool down period of about 3 minutes to prevent overheating of the combustion chamber. the following is from the manufacture on shut down procedure.

3.2.8Check Stopping of Generating Set
(1)Disconnect the load from the generating set.
(2)Turn off the air switch of the generating set.
(3)Set the speed lever at RUN position, run the generating set without load for about 3 minutes. Do not stop the engine suddenly, otherwise the temperature will increase abnormally, the nozzle will blocked and the generating set will be damaged.
Press down the stop lever. Concerning the electric starter, turn the key to OFF. Turn the fuel **** lever to S position.
Pull out the recoil starter handle until you feel resistance (at this position, both intake/exhaust valve are closed.) and leave the handle in this position. This prevents the engine from rust.
If the engine keep on running even after the speed lever is placed at the STOP position, either turn the fuel **** to the CLOSE position or loosen the nut of high pressure fuel pipe on the pump side to stop the engine.
Do not stop the engine with the decompression lever.
Do not stop the generating set with load. Stop it after the load removed.

the following link provides the complete maintenance manual for the diesel generator
http://www.rcremers.nl/webshop/handleidingen?download=20:handleiding-aggregaat

What exactly is the question?

Regular maintenance for generator is necessary that can help prolong the service life of the generator.

Generator Maintenance Tips
Generators
Generator Set Maintenance Tips
Useful Maintenance Tips for Diesel Generator Set

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Pump

Turn it off?

Seriously, you can try upgrading the exhaust (search for super trapp), or build a small enclosure to go around it (but still provide plenty of air flow).

BioDiesel has been known to quiet injector noise, but double check with manufacturer first. Howell's Diesel supplement can also help with injector noise, and keeps fuel from gelling in winter, has an algaecide in it, and is decent for fuel storage.

Low frequency means the RPM of the motor has fallen off.

Check your fuel is NOT contaminated or fuel filters plugged.

Further tests should focus on the govenor system. Possibly it fails when engine gets hot. the frequency is directly proportional to the engine speed.