Strange rattle when accelerating

The engine was rebuilt once, my nan doesn't remember for what reason though, it runs perfectly, I'll check if the deposits are oily. In the years I've driven the car it has never overheated, even while driven hard and with the old coolant. The first time I changed the spark plugs they were normal.
The engine has 172k Kms/ 110k miles and I don't know how many it's been since the rebuild.
The exhaust smells perfect once at temp, it doesn't smell like gas or steam.

As for the rattle, I think it's that too, it sounds like it's coming from the intake valves or something, I'll check the clearances once I get some tools, I already have the rocker cover gasket.

Whilst it probably is the head gasket failing, as I had a similar deposit on mine, I've read that yellow deposits in the coolant can be caused by repeated short journeys, something to do with it not getting up to temperature.

The only way to be somewhat certain that the head gasket has failed is to test the coolant for exhaust gases, one of these kits and it was soon off to the garage for an expensive job!