Why does a cooling motorcycle make popping noises? I was sitting in my parked car tonight when a Harley-Davidson motorcycle parked next to me. With my window down, I could hear that as the motorcycle was cooling down, it was giving off some type of “popping noises.” My first guess was that as the metal of the exhaust pipes cooled, thermal contraction occurs such that it produces some sound (even though I really have no idea why this would be true). I am hoping someone could correctly explain the cause of this popping noise.  
 A: A lot of materials make cooling sounds and also heating sounds, though you would not hear it with the motorcycle :).
Here is my hypothesis:
Materials that have an organized lattice structure, i.e.  are not amorphous, like metal, can interact, absorb and radiate, at a lot of frequencies, also at acoustical frequencies.
Cooling reduces the dimensions of the lattice  incrementally. As the metal radiates heat away ( black body radiation) different parts of it will radiate differently due to the geometry of the material. This induces stresses in the lattice, part of it supercooled with respect to another, and the balance is achieved by large parts of the lattice releasing  energy with  relocation in space , which creates pressure differences and thus sound.
Lattices have quantum mechanical solutions which include phonons 

The concept of phonons was introduced in 1932 by Russian physicist Igor Tamm. The name phonon comes from the Greek word φωνή (phonē), which translates as sound or voice because long-wavelength phonons give rise to sound.

