Pulse the bread in a food processor until it is like breadcrumbs. Mix breadcrumbs, onion, beef, egg white, parsley, garlic, salt, pepper, and parmesan just until well combined. Do not overmix.
Form into balls a little larger than a golf ball. This should make 8-12 depending on how big you make them. You don't want them too big or the tacos will be harder to eat. Imagine fitting 2 or 3 into an 8 inch taco shell. They will be soft and may not hold a perfect sphere.
Program Instant Pot for sauté. Add oil. Once oil is hot, carefully add meatballs and brown on all sides.
Pour marinara over meatballs. Lock lid. Set valve to sealing. Program for manual-high and 15 minutes.
Once the cook time has stopped and machine has beeped, allow the pressure to release naturally for 10 minutes.
Release any remaining pressure in the IP by moving the valve to venting (use a wooden spoon so as not to burn yourself).
Serve sauce and meatballs in noodle taco shells.