1996 Subaru Legacy Outback (sudden sounds change)

First of all thank you for a great resource, EricTheCarGuy, of course, and friends.

1996 Subaru Legacy Outback all of a sudden sounds different.
Low deep sound when you start the engine and idle. Sounds like a WRX or STI and not like an old school Station Wagon. This low pitch sound disappears to some extent when the car changes to upper gears ( low sound is really audible when in idle or the 1st and even 2 gear, when the car is gaining speed or climbs the hill).

Please help, if you possibly can. I would really appreciate it
I have a couple clues of what is wrong but I’m not sure…
1. Could this be an exhaust flexible joint (gasket) leak and the spring-bolt assembly should be tightened… (I think I can smell some exhaust in the mid part of the car when I open front seat windows… not sure though)
or
2. Could this be a transmission problem ( I don’t hear any suspicious noises from it though and it works pretty well except for sometimes takes longer times to switch into the 4th gear)…

So my main concern is that the car is just too loud, what could it be?
Please help!

This car has almost 150,000 miles on it.
It has a 2,5 L engine, automatic transmission
No major issues in the past. It’s used in New England so it’s exposed to pretty rough conditions (salt), sometimes really horrible highways (multiple bumps, frost heaves, etc.)

Thank you ever so much!

PS: Motor Oil level is F, color Ok,
Transmission fluid level F, color, consistency Ok (Cherry Cola)
Very minor leak (a drop or so a week from the valve gasket)
a little “oil like” stain on the rear differential box, but nothing is leaking.