Nothing wrong with back-to-back rads, Lisa. It sounds like a studwork wall (timber studs and plasterboard.) Possibly even metal studs.
The pipework is expanding and contracting as the others have said. It slides through holes in the studwork. Possibly, the holes are too small, but mainly it's the fixings that can cause creaking.
Holes in timber should be big enough for rigid pipework to pass through loosely, and fixings should not be rigid. Solid, standard pipe clips are the worst. Pipework should be able to slide around a bit as it expands.
If you can fairly accurately identify where the pipework runs, then it can be fixed. Just a case of cutting an access hole in the plasterboard to get at any creaking fittings.
Re-fix the plasterboard, then it's just a bit of Polyfilling or plastering to finish it off.