Strange Noises under hood 2000 LHS

Ok, I need some help/advice..... i have had my 2000 chrysler lhs for about 6 months now and within the last 2 weeks, i have noticed/heard a strange grinding noise coming from under the hood. problem is, it only lasts about 20 seconds and is only when i press on the gas. it also only happens when the engine is completely cold, like i have left it overnight. Oil level and appearance is ok and engine power is good. No leaks or other weird sounds. I really don't want to take it anywhere, since it is not a constant problem.... just need some ideas on where to start problem solving on my own. Any suggestions??