I have a 2003 Ford Mustang (standard, not a GT). She’s run like a champ and I take very good care of her, but the past two years, I’ve noticed increased clatter/rattle coming from the engine at the “bottom” (lower rpms) of the gears, but only mainly during the summer when the engine is very hot (I live in AZ). A friend suggested higher octane fuel and a quart or two of STP Oil Treatment, which I did. I haven’t noticed an improvement with the octane upgrade, but I did with the oil treatment - possibly my oil is breaking down faster in the hotter weather? Another friend wondered if my timing belt/chain needed adjustment. I want to take her into the shop for a check-up because I don’t want to keep dumping oil treatment into her, but I thought I’d look for advice here first.
Try a oil change with a good full synthetic oil. If it specifies 10W20 or some such thing, bump up the second number one grade. See what happens.