I’m writing to you because I bought an electric generator infrom your company, and after three days of used,age I noticed an odd noise while working it.

From rather than in.you don’t need the it as the sentence is all about the generator.

I decided to stop makusing it workso as to avoid any unexpected damage on it.

More natural sounding.

According to the guarantees per the included instructions, I have placed the generator in a dry area and only have used the specified oil and fuel given in the instructions.

The guarantee and instructions are separate things. The guarantee is the company promising to honour any purchase while the instructions are simply that, directions of use.

I’m writing to you because I bought an electric generator infrom your company, and after three days of useding it, I noticed an odd noise while working itit was on.

I decided to stop makusing it work to avoid an unexpected damage on itpossibly damaging the machine further.

According to the guarantee, I have placed the generator in a dry area and only havehave only used the oil and fuel givenrecommended in the instructions.

Please let me know how I canshould proceed to repair this product.

Customer sService

It's a minor issue, but you might want to observe proper title capitalisation.

I’m writing to you because I bought an electric generator infrom your company, and after three days of used, I noticed an odd noise while working it.

"Used" is a verb. "Use" is the noun.

I decided to stop making it workturn it off to avoid an unexpected damage ton it.

(1) "Stop making it work" is an unnatural expression. The phrase you're looking for is "turn it off".
(2) "Damage" is an uncountable noun.
(3) The phrase is "damage to (something)". Alternatively: "to avoid unexpectedly damaging it."

According to the guarantee, I have placed the generator in a dry area and only have used the oil and fuel given in the instructions.

A guarantee is an assurance that something will be repaired or replaced if it is not of a certain quality. In this context, I'm not too sure it's relevant.

Please let me know how I can proceed to repair this product (according to the guarantee).

If you want to mention the guarantee, here would be the place to do so.