Here is a softer variant:

    play -c2 -n synth whitenoise band -n 100 24 band -n 300 100 gain +20

This is the warp core idling.

I'm trying to add a slow warp to it, and after some experiment I got something. That's the first time I've used sox to generate audio, didn't even know it could. I'll play with it some more.

play -c2 -n synth whitenoise band -n 100 24 band -n 300 100 gain +20 phaser 0.5 0.3 2 0.8 0.5 -t

Sox can be pretty handy. For example it's easy to use it for musical instruments tuning (instead of buying some expensive electrical device).

For example for mandolin layout:

4 (G):

    while true; do play -n synth pluck G3 fade 0 3 vol 0.4; sleep 1; done

3 (D):

    while true; do play -n synth pluck D4 fade 0 3 vol 0.4; sleep 1; done

2 (A):

    while true; do play -n synth pluck A4 fade 0 3 vol 0.4; sleep 1; done

1 (E):

    while true; do play -n synth pluck E5 fade 0 3 vol 0.4; sleep 1; done

This is great, thanks!