It very much sounds like head gasket failure.

whilst HG failure is common amoungst these Rover engines the most common cause in the inlet manifold leaking water at number 4 cylinder, causing water loss and engine damge through water loss.

The hissing may well be water & steam escaping from the inlt manifold where it joins the cylinder head.

I hate to tell you but the moment you heard the rattling you should of stopped, noteven a few hundred yards more down the road. The damage is done is seconds.

secondly the resovoir water caps tend to be defective on these cars and need regular replacement, if they are not replaced they loose pressure causing the water to boil in the head when you stop & then once again head damage through hot spots.

Check the manifold to see if the steam and water hissing out change tine when you try and wiggle the manifold, but watch out for hot steam its dangerous.

Time to have it put down, methinks

Some people are buying cars like the rover because while they are ok, they last a while,like most cars they should be looked after ie regular checks made to water oil and brake/steering fluid.

But a lot of drivers buy untill it goes wrong then scrap, which at the prices rovers can be found for makes sense,I know several people that wont even look under the bonnet, and some dont even know if they have a spare wheel, and its becoming more common to hear of people that dont care about a car as long as it gets them from a to b,younger drivers are as bad