The other day I was offroading and on the way home my exhust was making rattles. It still does now too. If I'm idling or cruising it doesn't make the sound. But under power it is really annoying. I think its coming from either the Y pipe, or the first muffler thingy (manual calls it a muffler, but I thought it would be a cat?)

How easy would it be to get this out? And stop it rattling!

SOunds like your cat is breaking up. What I did on my EA81 was cut a three sided hole in the top of the cat and peeled it back, ripped everything out and folded it back down and welded it back. Now if you don't have access to metal working tools, you could take the Y off and get a semi flexible piece of rod and slide it in the back of the pipe and smash the crap out. This made it sound more hollow, I ended up cutting the cat out and welding pipe there, to finish the Y.

 

DISCLAIMER: Tampering with Emissions control pieces may be breaking laws in your country. (And not having a catylitic convertor will cause you to not pass emissions.)