weird noise coming from maybe cats

[bigcountry]

ok so this noise has gone away yet ive already gone to the ford sterlership to get both cats replaced pirce FREE!!! cant beat that and hoping that would solve this strange ticking noise i was getting from them but after a week or two the sound is back and it is still coming from the bottom of the motor towards the back near the firewall atleast now my cats arent ticking like the old ones were so what could this be from im at a loss ive checked valves not them nothing in oil pan everythings is tight im at a loss its loud never has been its not really notiable unless you listen for it and of course now that i know what it sounds like i always hear my fiance cant really hear it my aunt owns a automotive repair shop and none of the mechanincs can find where the sound is coming from. what do yall think the noise is coming from