Rattling sound at 1,600 rpm?

2011 ford focus with 100k miles, at exactly 1,500-1,600 rpms there's a strange rattling sound coming from under the car near the front. I think its possibly something in the cat thats loose but i don't know. Any help is appriciated. Btw it always happens evertime i hit that rpm

Easy to check if on a car hoist. Unless we can borrow for few mins? Then we need to try on ramps ( ensuring wheels etc chocked) so we can look underneath and better see noise source. A long stick such a broomstick handle can act as listening device. Place one end on heat sheild. Cat an other careful on ear. Rev engine until we get noise.
from sun dod it a loose /failed cat brick is the answer. Thus we may also when ( cold engine best) be able to reach under and 'shake' exhaust. We ought to also check for broken brackets? Heat shield fittings etc at same time.

Only an inspection of the underside of the car will find that. Yes could be a heatshield on the cat but could be other things and almost certainly nothing big or expensive. Until someone gets under there though it will not be found.

Get down under the vehicle when it's not hot and tap on all of the exhaust system heat shields. You're likely to find a loose one. You can put a radiator hose clamp on them to secure them to the pipe again or just remove them.

Try looking and tapping metal parts under a cooled car ( so you don't get burned from a hot part ) heat shield and any other metal parts… If car is running and the noise is in the engine then have a mechanic inspect your problem

Check the heat shield on the catalytic converter. They sometimes break-loose & rattle.