Adding Notes for User's eyes only

Is there anyway to add notes to a time entry that do not show up on the invoice? I often have the need to enter a note wherein part of it I want to show on the invoice, whereas other parts are for me only. I realize I can always adjust the invoice once created, but would rather not go through each invoice to find the undesired notes. Instead, would be great if there was a way to designated this directly in the time entry itself.