Toyota Echo sounds like a diesel tractor! on 2002 Toyota Echo

My Echo engine has been very quiet since I got it, but about a month ago very occasionally it would sound like an old dump truck or tractor for just a second or two and then the sound would disappear. I thought I was hearing things at first or actually hearing a vehicle next to me. Then the sound lasted longer and longer. Now I hear the sound about 10 -20 % of the time the car is runnning, mostly when we are idleing at a light or just starting to accelerate. Any ideas what it might be. I'm taking it to a mechanic this week and I want to have an idea what to expect. JD

Asked by for the 2002 Toyota Echo
I'd say its likely to be a cracked exhaust manifold or leaking exhaust manifold gasket, it would appear intermittent perhaps because the leak may "seal itself" because with heat the metals would expand and seal. Hopefully it is not an internal lubrication problem in the engine.
Thanks Patrick - a friend suggested that I might actually be low on oil. I will be trying to top it up today and see if that makes a difference. JD