Seeing spurious peaks in acquired vibration data; peak frequency changes with sampling rate I am acquiring vibration data from an accelerometer using a DAQ. Due to some reason I am seeing spurious peaks/impacts in the FFT spectra at specific frequency and its harmonics all the way till the Nyquist frequency. This frequency changes with the sampling rate as follows:

Sampling rate = 48 KHz, peaks at 100 Hz and its harmonics
Sampling rate = 8 KHz, peaks at 83.33 Hz and its harmonics
Sampling rate = 24 KHz, peaks at 93.75 Hz and its harmonics

These peaks don't seem to correspond to any real mechanical/vibration source. Initially I suspected aliasing, but it seems the DAQ has anti-aliasing built-in. I also tried doing anti-aliasing in software using python/scipy.signal.decimate(), but it didn't help.
I am attaching below couple of FFT spectra plots for the 24 KHz case.
Can you please advise what could be possible reason for such spurious FFT peaks?
Can you please advise what could be possible reason for such spurious FFT peaks?

Most likely it's your measurement setup. Good  measurements are hard to make and require careful setup, characterization, and execution.

Determine the noise floor (spectral shape and level) of your data acquisition system, just short the input and grab few spectra. If that looks good
Determine the noise floor (spectral shape and level) of you sensor: attach the sensor to something heavy that's not moving and grab few spectra. If that looks good
Build a &quot;calibration&quot; setup. Attach your sensor to a well characterized, calibrated or programmable vibration source (a shaker, or maybe a electro mechanical driver) and measure different frequencies at different levels and compare extend the expected results. Determine the frequency and dynamic limits of your setup.

In each of the steps above set targets, requirements, and expectations BEFORE you make the measurement. These should come from the data sheets, general physical considerations and the specifics of your application.
Don't even look at any real measurement data until you have proven that your setup does indeed measure vibration with sufficient accuracy and signal to noise ratio over the required frequency range and dynamic range. If that all checks out and you still see unexpected data you can ask again.