[Rick Lafford]

Can be an indicator of poor fuel, leaking head gasket or fowled spark plugs due to the EGR valve passing oil vapor back to the intake. To check for a leaky head gasket, mark the level of coolant in the header tank, drive for at least a tankfull of fuel and then check the level again. If it hasn't changed than your head gasket is probably OK. Is the vehicle using any oil?

 

[Rick Lafford]

If it's only happening on a cold start then it was probably due to a weak spark on one or more cylinders. Mine did same (lots of clatter from the hybrid transmission drive coupling) until I changed plugs. As for coolant, I recommend that you mark the coolant tank level and watch it over some span of time. The head gasket problem is well known and worth catching earlier rather than later. There is a service bulliten and updated EGR valve (with programming) available. Cleaning the intake manifold is difficult but a new part isn't prohibitively expensive, nor is the EGR valve.