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Noise as observed on an HPLC Chromatogram is the summation of various components, which end up superimposed upon each other.

Electronic Noise – comes from the Mains electricity, or induced signals from cables running together or RF signals from other instruments 

Pump Noise – a much longer frequency noise, corresponding to the check valve opening and closing each pump stroke. Pump noise changes with flow rate, and stops when the pump stops 

Lamp Noise – the output from the lamp includes microscopic flickering etc, and this gets worse as the lamp gets older. This is high frequency noise, and can include spikes 

Air in the Flowcell causes noise. This can be quite high frequency and can be very large 

Mixing of solvents from a gradient pump causes very low frequency noise at the speed of the proportioning valve cycle. This can be improved by changing the cycle time and eliminated altogether by pre-mixing for isocratic runs. 

Temperature variations also cause noise, as the column heater controls temperature. An old style thermostat give huge, low frequency changes, while a more modern device which pulses in the power will generate little noise.

Under good operating conditions, noise should be no more than about 20-30uV, and should be a combination of the above components with none of them dominant. In the event that the noise increases, it is likely that it will be one of the above that causes the problem, and hence the noise pattern will alter.