I was wrong, it was the Red one!
Your two responses saved me alot of unnecessary work. After a bit of wriggling, front duct separated enough for me to reach in and find the offending pen. A bit of digital gymnastics to get it properly oriented for extraction and I had it. No need to remove rear duct, but helpful to know had the front not snapped back into place.
Thanks...I'm now rattle-free.
'75 Triumph TR 6