Furnace vent is roaring like an engine

[affiliateian]

Just noticed that when our furnace comes on, there is a distinct "engine roar" sound. Upon investigation, if you put your ear up to the wall where the vent line goes up to the roof (from the furnace), it is definitely this wall that the new noise is coming from. Furnace and home is 3 yrs old, nothing different this winter except we had a central A/C put in over the summer. Just changed the filter and noise is still there. What could causing this roaring sound?

[JoeSpareBedroom]

Some sort of obstruction, maybe? Creature? Bird's nest? Is the vent capped to prevent this sort of thing?

[C & E]

Ditto that. Air attempting to rush through a narrowed opening (venturi?) makes noise. Think, air rushing in through a crack around a door - maybe you're too young to remember the days before weather-tight homes.

[Art]

Also look for a plumbing pipe that might be touching a exhaust or intake pipe. They vibrate when the furnace starts up and if a plumbing pipe settled against it that could make a huge racket. Been there and fixed that. Put a sponge in between if necessary.