After reading the posts up to now, I agree that the chances are that you have a leak at the point where the exhaust manifold is connected to the header pipe. BTDT.

Why the noise only when accelerating? My guess is that the engine, which will normally move a little side-to-side on acceleration, moves just enough to open a crack which otherwise stays closed. You should check the condition of the right motor mount, too. Weak mount allows more engine movement.

Cold engine, use hand to feel around where the manifold bolts to the header pipe. Have a helper in the cabin, car in Park, rev the engine and feel and listen.

Not a complicated repair, just a PITA. New Volvo (or equal quality) gasket, proper fitting wrench (use a 6-point socket if possible), a use P'Blaster to soak the nuts. Let it soak in a good while, those nuts get hot, rust and are not easy to remove.

Before all that, be sure to look about at the other locations for the leak.

Dunno about using any sealer. There might be an exhaust sealer that could help. Ask the other posters about that.

Good Luck,

Bob

:>)