It's not really a specific tone. Rather simply vocally mimicking the tones you hear wile in the trip, that ringing sound you hear. It's just about matching the pitch so even just humming the tone works.

I wish I could tell you which McKenna recording I heard this on, but I have like 40-50 hours worth so it would be some serious work trying to find it :d 

I think the reason this idea made an impression on me is because a similar technique can be used in a form of energy healing I've learned called Quantum Touch to change the intention or "flavor?" of the energy.

I tried it on my first DMT trip during the mandala pattern part. Mimicking the tones I heard changed the shape and color of the patterns I was watching in this stage very obviously.

If your interested you should try it on your next trip (during the sub breakthrough parts. I'm not sure how it works or even if you would remember to try during a breakthrough.) and let us know what it does for you. I would love to hear your experiences.

If you have any other questions about it I'll try to help  enjoy!