Is this a shop-made boring bar, or a purchased unit?  Is it rotated to provide clearance?  I found this out recently.  My purchased bar is rotated to provide cutting tip clearance, and my shop-made bar rubs from lack of clearance.

If you had 3″ of work sticking out the chuck, how much boring bar did you have sticking out the toolpost. Generally the more tool stickout the more likely you are to get a high pitch chatter which sounds like the tool is screaching.

If it is more a graunching noise then look to see if part of the holder is rubbing or the bottom edge of the cutter bit/insert.

Not taking enough of with a general purpose insert can also have it riding out of the cut try a **GT insert if you are using an insert tool with small corner radius if not a blunt or poorly ground HSS one will act the same.

If it is possible, reduce the speed. Always a first try when chatter happens.

Boring bars are thin, and flexible, the more unsupprted length, the more flexible

(Assumes, as said that the tool is sharp and mounted with the cutting edge on centre height.  If it is not, you are on a hiding to nowhere!)

Sometimes, a light cut will chatter when a heavier one will keep the tool “dug in”.

Currently, I am boring a 3 inch long brass bush.  It means noting the position relative to Jaw 1 of the 3 jaw, ready to use the same position when boring from the other end, to maximise concentricity.

Fortunately it is a slack tolerance job!

Not ideal, but when needs must!

If there is room, you could try wrapping the shank of the boring bar in lead, to lower the natural frequency, and to damp vibration.

Howard