Check the valves, the valve lifter, and the oil pump.

ya  mine was doing that.. it's your lifter.. don't let that go or one day it will get really really loud all at once  and then about 10 min later send a push rod through the block

It COULD be a noisy lifter, but it could be also something as simple as a small exhaust leak.

It could be poor fuel , causing a "ping" or slight "knock" from the motor . Try a higher grade of fuel.
  If the oil is low, or dirty ( over 3000 miles since last oil change) you might need to change it.
Sometimes switching to a synthetic oil , or synthetic blend will help make the motor run quieter . It costs more then regular oil , but it is far better for the engine . ( this is a cheaper attempt to get rid of an engine noise, instead of replacing lifters )
  Lastly , a small exhaust leak will make a "ticking" noise too....usually when the engine is first started up an put into gear , or under heavy acceleration .
  I would go to a good repair garage to see if they can diagnose the source of the noise , and recommend what actions to take .
If the engine has high mileage it is probably getting tired and may need an oil pump, lifters , etc... ...especially if it has been neglected ( no maintenance ) so if you are handed a big estimate , shop around for a couple of opinions . Stick to a smaller garage wit ha good reputation.

If it is a lifter tapping in you motor then the next time that you put oil in it put a quart of Lucas oil stabilizer and c if that works.

it may be a cracked header. Do you smell gas and is the idle steady or not?

bad lifter, stretched rod maybe

ford are known for this ticking sound it is a lifter problem alot of the times the valve guide gets stuck and binds up.. it is a big fix but if it is still a new noise there is still a solution that may or may not help but worth a try... buy some power steering fluid and drop a liter into the engine...
i know it sounds stupid but it works, the p/s fluid and oil don't mix and the p/s fluid will help lube up the valves, some might say trans fluid but p/s fluid works by far the best ...

have yourself a great day.....

agree with dave on this one...a litre or quart of power steering will stop this annoying noise...have done it on quite a few occassions myself and surprising it works....hope this helps

that would have been your rod bearing not a lifter a lifter would just be noisey

yeah  probabally a lifter issue...  but it could also be a exhaust issue...   i just got done with the latter...   few holes from winter driving and im hearing a tick...   fixed the hole no more sound...   interesting with the power steering fluid guys... im gonna look into that ...   thx...

i have seen that alot i work on alot of stuff like that it tends to be the piston slapping due to wear do u change the oil regularly and try the cylinder restore that u can buy buy in most cases it wont get better

yea i guss u know by now that its the lifter.... they sell some stuff called marvol mistery oil its in a red bottle add some to ur oil and it shuld stop within a few days of driving it...

you can try to dump as many different things in the motor as you want if its exhaust this will not help if its a lifter it may make it quiter for a short period of time but will deffinatly not fix anything.  If you are going to start pouring misc. fluids into your motor do yourself a favor and change your oil and flush all the crap you just poured in out because it won't ever fix anything just make it quiter for a little bit then it will get worse bring it to a shop if you are unsure and have it fixed before it developes into something worse it just started fix it and don't listen to anyone who tells you to pour crap into your motor thats a backyard fix

or just get something better then a mustang, :D

check valves and valve lifter....and it could be your exhaust manifold gaskets

you drive a ford dude, you need a new head.

that probly means tha rods n pistons r knockin

try to change ur oil and put in a oil addittive to stop it or it has to b rebuilt

it could just be an exhaust manifold leak  it can sound like a rod knock also make sure you got enough oil too

tappin valves

i would take everybodys advice on the lifter and con-rod. get it seen too professionaly as it can mean a very large bang and then your engine is damaged.

I would check your oil pressure, and change your oil. then see if the sound goes away, ...does the knocking come from the top of the engine? do not run Penzoil in your car, it sludged my oil pump up on my 03' mustang and it started knocking bad, i had to buy new rod bearings and a oil pump..., but that is my opinion on what you should do is check your oil and change it...then if its still the same get it checked out, autozone will tell you what is wrong with it for free, but they charge you to fix it.