Why does my car sounds like a diesel engine when cold?

This problem has been going on for quite a while. I have a 1998 Nissan Maxima (110,000 miles) and trying to fix it up. The sound usually happens in the morning when cold but goes away once warm. I have a few problems that could be results. Bad water pump? Piston slap? Help would really be appreciated. I need the car for work (not traveling far) so how can I maintain it enough to get my by until I get enough money to fix it.