Exhaust rattling

About a fortnight ago my car started rattling, it does it in every gear at 2,500 RPM roughly. It doesn't do it while not moving. Hence this is proving tricky to find the problem. It is smooth up until these revs then sounds like something is loose but only lasts for about 200RPM at max between the acceleration. It sounds a lot worse when the window is down and as summer is fast approaching I want to get it sorted ASAP.

Had it in to a garage and had the heatshield checked, the technician then said he 'thinks' it may be that the cat has collapsed but would need to take it to bits to find this out.

Does anyone have any other thoughts as I am reluctant to go to the garage again after having paid these cowboys to look at this the first time round

Thanks
 
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