l need to know the technical details or any other that can explain a royal enfield bullet is thump-thump noise..............

Single cylinder bike. Sounds just like the old BSA 441 is from the '60s

Royal Enfield?
Is not that where the queen keeps her chickens?

it is simply the CC of the engine, coupled with the exhaust valve and the diameter, material content of the exhaust pipe and leangth.

Now just to straighten you out, It aint all that. l had a briggs and stratton engine that sounded the same.

Go with 'themanintheyellowhat'. he knows of what he speaks.
Single cylinder big bore bikes have a sound all of their own. l have a 1968 BSA 441 Shooting Star and it has the sweetest sound this side of Heaven. And that is with the stock exhaust. That is, when l can get it to run. LOL.

its a 500cc thumper. they sound unique. like half a harley.

i love big singles. l started with a matchless g80s, then l had a bsa, (sadly not a goldstar) then an Xt 500 yamaha, Ft500 honda (piece of junk) and an Sr 500 yam, which l chopped.

one clylinder, 2 valves, massive induction roar, great gobs of torque, and am exhaust note that scared teh cattle 3 fields away.