To all:

Seems I found my root cause. I have Backblaze cloud backup running on my computer, which, once an hour, backs up any changed files to their server, hogging all the upload bandwidth. Even though there is supposed throttling of backblaze bandwidth, it doesn't seem to work effectively.

I was able to have a call with my wife, start the backup and get the pulsing sound. I would stop the backup during the call and the pulsing disappears. 100% correlated!

Woo hoo! But now I have to deal with backblaze!

This did not happen with Linksys/ITPVoip combo before Voipo, so I don't know why Voipo is so much more affected by this, but at least I found it!