lost power with a droning noise on acceleration

Hi, my 2001 Jimny hardtop has lost power relatively suddenly (Its my daughters drive so I'm not sure quite when it happened) As well as the loss of power there is a droning sound when the accelerator is fully open and it sounds like its coming from the air cleaner box, i removed the filter in case that was the issue but no change in performance or noise.
Any ideas anyone???

thanks

David

My Jimny seems to have less acceleration and also has a pulsing whine that only appears when accelerating, though it sounds to me like its gearbox or transfer box, I plan to get it booked in to the dealers to see if they can diagnose the problem.
Gaz

Have a feel at the pressure coming out the exhaust
could have a collapsed cat or silencer just a thought

Jack

I disconnected the small breather hose from the rocker cover that goes in to the air cleaner and blew blew down it, I could hear air coming back into the air inlet, It wasnt an easy blow through and wen I released the pressure there was blow back out of the pipe.

It was a collapsed Catalytic converter, many thanks.