You might not be looking for vintage synth sounds however Easy Sounds is coming out with "Vintage Keys" converted for Montage in March. You can review the data list of the Motif Vintage Keys version to see if anything there is of interest. Sorry if someone has already mentioned this.

My interest in old synth sounds is more basic. I am looking for white noise - I don't find it in the waves list. If I remember correctly, the DX7-2 had a white noise generator, which could be taken to mean the the FMX engine might have one hiding in there somewhere....

See: https://www.yamahasynth.com/ask-a-question/my-biggest-problem-with-montage-monophonic-lfo-seriously

I was able to make noise with the LFO by setting the rate high and affecting pitch as the modulated parameter. I also had the part LFO drive the element LFO. It's not 100% noise, but very close. Certainly becomes closer with 2+ notes at once. But for a burst of noise - one note should be fine.

My normal way to share audio clips is not working at the moment - but I have a demo queued up.

Part LFO - triangle
Tempo sync ON (tempo=300), Speed = 1/16th, loop=on. Destination Element LFO, depth=127, element 1 ratio =127

Just have one element active (element 1)

Element 1 LFO - triangle - speed 63 (although I recorded with speed=38) pitch mod = 127

Not sure it matters so much what the sample is - but I had one of the P5 Saw waveforms on element 1 left over from some previous work.

... and a demo
https://clyp.it/4fx21dxc

My interest in old synth sounds is more basic. I am looking for white noise - I don't find it in the waves list. If I remember correctly, the DX7-2 had a white noise generator, which could be taken to mean the the FMX engine might have one hiding in there somewhere....

My Montage and MODX have a wave called "White Noise" which sounds surprisingly like white noise. Did you not find it or is it not good enough for some reason?