I'd vote it's a leak from the exhaust manifold. I've had two cars develop this problem, where you could here a ticking noise coming from the engine, like a valve train noise.
In one car the manifold was cracked.
In another car (the Nissan), the studs on the manifold had broke, a common problem on all Nissan's with the 3.0 liter V6.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".