Take the belt off the mower deck and now check the spindle for any play. While the belt is off, turn the bade, do you hear the scraping noise? If no noise, follow the belt back to the engine and look for anything that is bent ( pulley, guards, etc.). Is the blade installed properly ( air-foil to the deck not to the ground).

I've pretty much decided it has to be the blade although its not a regular shake like an unbalanced blade. I have a new blade I could put on it to compare so I guess I'll do that first. The old blade has 5 seasons on it and I took it off and sharpened/balanced it 2 years ago.

I removed the blade, which looked great and was in-balance. It was still sharp enough to cut if you grabbed it wrong. Then I started the mower with no blade, engaged blade drive and it was perfectly smooth, which indicates the blade being at fault. So i put the new one on after balancing it - had to use the bench grinder - I remove steel for balancing by sharpening the low end. So, I put it on and its better but the deck still shakes. At max engine speed it settles down and doesn't visibly shake. So I guess I'm OK with it but I still don't know how it was the blade when its not damaged and checked to be in balance. Whatever, I did not throw it away.

Used the mower today and although the deck still shakes a little, at standard cutting throttle position (fast) there's no problem. 'Shrug', move on. :)

Oh yeah, I found out that there is a blade brake that engages when you disengage the blade drive, so that explains the scraping sound when manually turning the blade with the drive off.