Noisy after a few hours - Getting too hot?

I apologise in advance for my clumsiness in describing this problem.

Basically I am on economy 7 and so the unit comes on over night to heat, for the first while it is at a reasonable noise level. Then it gets very noisy, it sounds almost like it is boiling (The length of time this takes seems to vary, possibly depending on how hot it is to start with. Typically after 4 or 5 hours if there was no heat left when it was turned on). I can not see anything that looks like a error light nor anything that seems like a temperature gauge. Any ideas what's wrong or if this is normal? Information on how hard to fix/costs would be good also, my granddad is an electrician so he could potentially do the work if it is not too complex?

Thanks for any help!

Check the rod stat/s
Replace if needed.
Also some boiler silencer added to the unit can quieten them down.
Mike
MGC/H2O
Norfolk

Thanks, Guess I will need to call someone out to look at it then.