Car holding back at 2k revs but sounds really boomy

Hi guys, I’ve had my clio for about 2 years now and this morning I was driving to work as usual got on an a road and went to put my foot down at about 2k revs and the car held back and went really boomy almost like I had no exhaust on the car but once it got over 3k it was perfectly fine. Now I am hoping I might have split the flexi but it in first gear it doesn’t do it so it’s quite odd and anything after 3k it’s fine as well still seems a bit more sluggish than before tho. Any help please!
 
Was the engine warm or cold when it did it? Which gear were you in? Might be best to try and get a video of it happening.
 
It was warm and cold and it did it in every gear other than first and yeah I’ll try get one tomorrow morning
I sadly found out it that I didn’t fix it at all it is still present, took one coil out at a time and can confirm it is not a misfire but I have some videos I will post the car only does it under load and it seems to sound like a tractor
 
Have a look at the exhaust manifold, pretty much everywhere from the headers back. Might be a leak / crack somewhere.