Heat exchanger restoration...?!

[ash_on_mtb]

I was 'lucky' enough to score an underbonnet shower recently, and I've pulled it down to clean it all up. Found something I need a hand with tho...

The pump etc was FULL of mud and crap, and it looks like the exchanger has been responsible for cleaning much of the silt out of the murray, but when you get it off a truck whose owner simply loves the bog holes, you'd expect that.

Question is, the heat exchanger is a plastic case one, and the copper coil seems to be brazed into the in and outlets to the motor loop, but there sounds like there is something loose in there. Flushing and shaking, I'm getting heaps of mud out of it, but the mud is starting to die down now, yet the noise is still there. It sounds kinda like a wet sock bashing into each end, so I wonder if there is something purposely put in there to make sure that deposits don't form inside!? Kinda like an agitator or something?

Any merit to that idea, or do you think its just mud and I need to keep flushing it?

ta.