Mahmut, a clean line in day time but arising noise at night suggests other environmental interferences which is hardest situation to cope with.
I am afraid, under this circumtances, neither you nor Telco have very much chance to fix it. Even worse, the ADSL, based on the idea of running multiple modems sharing the same physical media(very roughly, frequency multiplexing) is much prone to noise. So, think twice when you to upgrade your modem.

As a customer, Yes you are absulatly right to have flawless service. But to be righteous doesnt necessarily make the things result in favor of you :o)

TELCOs have rather sofhisticated diagnostic tools either inherent to the CO your line is connected to, or other test tools. If you make them angry enaugh, believe me they can find out your location.

Best regards.

Rustu