You need to post the model number of your engine as there is more than one 18.5 hp B&S engine and they are drastically different.

Walt Conner

I believe its a 18.5 HP* LT 42 Tractor. 18.5 HP Briggs & Stratton® OHV Engine with Oil Filter

Bought from Tractor Supply Co. about 5 years ago.

you'll need to look closely at the sheet metal on your engine - will find either a decal/plate with model/type info or the info will be stamped into the shroud sheet metal.

Look on the engine itself to find 2 critical numbers.
The model number and the type number.
Look at the example photo below.
Yours might appear to be a little different in style but it will have all the numbers as in the example.
Need model and type number.

{{gwi:325617}}

Maybe you can't find the model and type number because the connecting rod broke it off the side of the engine??

Considering the circumstances (all of them) I suspect that during the de-railment of the belt, a chunk of material was torn off the ground drive belt (transmission). Now, as the gapped belt section passes each V-groove pulley.......it produces a bang or thud. It is not uncommon for a belt (or pieces of a broken belt) to be thrown from the pulleys of one system..........into the belts of another system, causing a cascade effect of damaged parts in the other system.
The fact that the noise persists even when the deck is not engaged is what sort of points to a chunk of material missing from the "V surface" of the transmission belt.

OR something got wedged in the V of the engine drive belt pulley. Got a service call once and there was part of a Hickory Nut shell wedged in the pulley.

Walt Conner

I would also take a close look at the belt (if you haven't replaced it) and the wedge alignment key on the crank. Is this thing manual clutch or elect.?