Computer case causing interference with TV
It sounds like your new case has a lot less metal than the old case, and is allowing signals from the CPU (and the motherboard in general) out. The CPU heatsink is closely coupled to the CPU chip, and acts as an antenna for those signals. Undoubtedly, the electrically-conductive parts of the conventional (air-based) cooler have different resonant frequencies than those of the water-based cooler, and this would explain the shift in frequency of the interference you're experiencing.