How did you make the marinara? I like to roast my tomatoes and deseed. The seeds can give the sauce a sour taste or metallic if blended in with the sauce. I use carrots as a natural sweetener. As they get sweet when roasted.

Add brown sugar

If you used unlined copper or aluminum, either would have given you a metallic taste. When cooking with any kind of acid, including milk and cream, always use stainless steel because it is non-reactive with acids.

Possibly you have high acid tomatoes and you just need a pinch or two of sugar? What kind of pot did you use?