I'd use the word moaning instead of onomatopoeia or panting.
From Oxford Dictionary; "moan"
A long, low sound made by a person expressing physical or mental suffering or sexual pleasure.
So your sentence becomes:
I'm tired of listening to your moaning every night.
In agreement with talrnu’s comment, I think “window shopper” is in fact appropriate here. In usage (which might be better represented on Urban Dictionary and Yahoo Answers), this term is commonly employed to refer to people who walk around both inside and outside of shops, and is used to differentiate such people from those who have more of an inclination to make a purchase.
From Urban Dictionary:
Someone who looks at stuff they can’t buy.
When one visits a store or mall to admire goods rather than to purchase them.
From Yahoo Answers:
It implies [. . .] someone who is not serious (a shopper who has no real intention of buying [. . .])
“Browser” does not sound right to my ear as it seems like an awkward nounification. Most people will think of something they use to view websites. I have not heard the word used this way. It would probably require explanation, which would defeat the purpose of having a single word for it.
That said, I have heard people say that they are “just browsing” to indicate that they are not actively pursuing a purchase at that moment. For your case, I could certainly see someone saying “he’s just browsing.” As far as saying “he’s not a customer”, you have another problem.
Anyone visiting the store is, potentially speaking, a customer. so it’s not ideal to say “he’s not a customer” unless you are pointing to an employee, security guard, pomeranian, etc. You might be clearer if you qualify the word “customer” before you establish this comparison, for example: “he’s not a serious customer, he’s _____________.”
Lastly, another term I know for such a person is to describe them as a looky-loo.
looky-loo
(also lookie-loo)
NOUN
INFORMAL
1.1 A person who seems interested in making a purchase, but whose actual intention is only to browse:
a treat for all the North Shore’s looky-loos: the popular Spring Designer Kitchen Tour
A good business broker will separate the real buyers from the looky-loos, bring in more qualified prospects, and usually can garner a better price for the business.
Source: Definition of looky-loo in Oxford Dictionaries
Best Answer
I can't think of any situation in which "I would like to ask about the function below" would be considered too informal; so it should be fine as it stands.
'Ask' should be considered neutral rather than being formal or informal in-and-of-itself; it is what you combine it with that determines how formal the sentence is as a whole.
However, if you want an alternative which does carry a connotation of formality, you can substitute 'inquire', as in:
'Enquire' is an alternative spelling of the same word which might come across slightly differently, depending on the audience; if you're unsure, 'inquire' is likely the safe bet.