Need help cleaning noise I have been recording sound sample from mouse vocalisations and noticed that a background noise appeared in the recording room. the noise is clearly visible in the spectrograms of the recordings as horizontal lines at frequencies 20, 40, 60, 80, and 100 kHz (ultrasonic range). I have tried to record in an acoustically isolated box, lined with sound absorbing material, and it helps, however, the box retains unwanted odours which effect the behaviour of the mice, rendering it unusable.

My question is if there is a method to isolate and remove the noise without affecting the rest of the sounds. I want to study the vocalisations, with an emphasis on the frequency in which the mice communicate under different social encounters, and having that constant noise in the background may affect the results of the study.

Vocal communications were recorded using a 1/4 inch microphone, connected to a preamplifier and an amplifier (Bruel &amp; Kjaer) from mice. Vocalisations were sampled at 250 kHz with a CED Micro 1401-3 (Cambridge Electronic Design Limited, Sunnyvale, CA).

The samples were recorded using spike2, a "DC removal" filter was added to the recording. the files were then transferred to Matlab for splicing further analyses.

I know that there is a strict policy here against attaching files to questions, therefor I'm adding a link to the audio sample and the spectrogram

link to an audio sample and a spectrogram via google drive.
I took a look at your noise spectrum and it's pretty bad 

You have strong lines at 20kHz and harmonics thereof. But also some spurs around  15 kHz, 18 kHz, 25 kHz and 44 kHz. The stuff at 20 you could potentially reduce with a comb filter, but your best bet (by far) is to clean up your measurement/recording setup.

There is no reason for this noise to be there, and you should try to identify the culprit(s) and eliminate them. Look at interconnects, other devices in the room especially lights or communication equipment, electric interference, power supplies, grounding issues etc. 

This will be very hard to clean up, so having a good recording in the first place would be much preferable.