This might sound backwards--and I'm not sure if it'd be everyone's cup of tea--but I've been doing ACHV Peak dumbbell workouts from You Tube, and as long as you're not doing immense weights and grunting, they keep the pace going pretty well. They can last 20-30min, giving a timer cue usually of 40sec working followed by 20sec. rest. You can do a couple back-to-back (although follow the instructions for good form to prevent back issues). And, you can start off with any handheld weight you want.
Note: If some heavy breathing towards the end is a problem, though, I'm kind of at a loss for how you could do any cardio with absolutely no noise.
PS I assume for some reason you've ruled out a brisk walk outside (by far the best exercise all around), swimming, biking, hiking, or (my favorite after a therapy session) walking up & down a multistory staircase several times.
_________________
"Engineer type" w/ ADHD (AQ:35-40, SQ:80, EQ:11-18, FQ:24, Aspie Quiz: ND 103/200, NT 100/200)
-Fan of Dr. Russel Barkley lectures (ADHD), "How to ADHD" toolbox tips, AttentionTalkVideo, Therapy in a Nutshell, and Mark Hutten M.A. (Asperger's) channels on You Tube.