Knocking?

Sorry if this has been answered before, I searched lots of threads but didn't find a specific answer.
I have been getting some knocking only when the engine is cold, although I don't hear it while idling only if i give it a little gas, up to around 2500 rpms then let it idle down. You only hear it when idling down not when revving up.
Once the engine is warmed, about to the first line it's gone. I was told this isn't a big deal but it really bothers me. It just started about 4 months ago and is really not very loud. Just want to know if I should be concerned?

The car is a 2000 mustang gt, 104000 mi., no mods ever, well maintained since I bought it in 2003.

Also it had 53000 mi. when I bought it but had to run on 91 octane gas or got a lot of detonation, dealership gave me all maintenance paperwork with it and said the Texas weather was the reason it needed the 91 octane which I know is . I have run 93 octane since I bought it.

I have plans to start modding it when the money becomes available, I'd love to s/c it. I'd like to get around 400hp with as little work as possible but If I have to rebuild the engine I will.

I have been told from other members that it probably had a tune, and I'm fine with running 93 octane gas in it but just wanted to know if the knocking I'm hearing will likely lead to more serious problems or if it is just normal as some friends have told me.

I also hear a loud ping sound around the engine about every 30-45 secs. Sounds like a hammer hitting the engine, I hear it any time the engine is running and for about 20 mins. after I turn it off, never noticed it before.
I know most cars make this sound after being driven for a while when the engine is hot but never heard it on any other car right after starting the engine.

Thanks for all replies, I appreciate it.

Thanks for the advice wohorsy2002, I believe that it is the timing chains. I have a couple questions now:
If I buy a timing chain set is the Cloyes one for $263 at autozone good, Summit has the same one for $275 plus shipping and I couldn't find any others?
I am thinking of swapping the cams while I'm at it but the comp cams I looked at say they require bolt on cam gears but I've read that 99-2000 gt's have press on gears.
Can i just buy a timing chain set for a 2001 and up with bolt on gears or is there a better way to do it?
My total budget will be around $900 with me and a friend doing all the work.
Thanks again everybody for the help.

Thanks for the replies.
The reason I was asking about using a kit for a 2001 and up is because it comes with both cam gears, guides, tensioners, crank gear and chains and I found one on Ebay for $157, says It's the same Cloyes set sold at all the other places but about $100 cheaper and new in the box.
The cam gears from Ford were $45 each and chains are $32 each.
So I would rather get a full kit even if I have to buy the one for $260.
That way whether it's the guides or chains(or both) I'm covered.
Just wanted to know if theres anything that would make the 2001-up kit not work on a 2000?