Clicking Noise

I have a 1998 Mustang V6. I noticed that there is a clicking noise when I use the gas. At first I just heard it when I was going slow through parking lots, and only when I pressed on the gas. When I would coast I would not hear anything abnormal. The noise would normally go away after I started going faster. I realized though after about a month of this that it may not be normal. So I put the engine in nuetral and pressed the gas slowly, it also clicked when I did this as well. It just is very quiet and you can not hear it very well, it apparently is drowned out with the noise of the engine as it goes faster, so is always there. I know that it is not the tires or the axels, but I do not know what it is. Does any one have an idea? I have limited that it is not part of the wheels or axels. Thanks

Does the clicking increase in frequency as you rev the engine? Or is it constant?

No, it does not make any noticeable increase.