Wenn's eine 1,9er Maschine ist, dann kommt leider auch dies hier in Betracht:
http://www.miata.net/garage/tsb/sb00-01-014r-808r2.pdf
Auszug:
A rattling noise may be heard from the engine when depressing the clutch or when revving the engine in neutral. In
extreme cases, customers may experience noise at all times. This concern can be caused by improperly machined
number 4 main cap of cylinder blocks which may result in premature wear of the thrust bearing.