Was it smoke or could it have been steam. One cause of a hollow sort of exhaust note is lack of cooling water. This would also give rise to steam.

If not that, then it could be more serious.

This is what Google says about white smoke

White smoke occurs when raw diesel comes through the exhaust completely intact and unburned. Some causes of this include…
Faulty or damaged injectors
Incorrect injection timing (could be a worn timing gear or damaged crankshaft keyway).
Low cylinder compression (eg caused by leaking or broken valves, piston ring sticking, cylinder and/or ring wear, or cylinder glaze)
When white smoke occurs at cold start, and then disappears as the engine warms up, the most common causes are fouling deposits around piston rings and/or cylinder glazing. Use of our Flushing Oil Concentrate and FTC Decarbonizer address these respective problems.

Water entering combustion spaces will also create white smoke. Faulty head gaskets and cracked cylinder heads or blocks are a common cause of water entry, and are often to blame. Unfortunately, expensive mechanical repair is the only proper solution here.

Good luck