Detonation, or Piston slap, or rod knock IMO.

Piston Slap:
Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.


Rod Knock
Rod knock isn't always apparent at idle due to the oil films ability to cushion the crank/rod. Rod knock will get worse/louder with rpm and isn't dependent on load at all so you can hear it while sitting still.

Detonation:
Detonation or spark knock is a bit more difficult to detect and occasionally can't be heard at all. At best it will have the effect of making the engine sound similar to a diesel or it can best be described like a handful of marbles rattling in a coffee can. Detonation in street vehicles will generally present itself in low rpm/low vacuum/high load/high temperature situations.
Sounds like classic rod knock. The oil isnt cushioning when cold since its thicker and the noise is probably horrible at startup.