rattle in engine compartment

[suziswift]

G'day all,
I have a 3 cyl 1993 Swift and it's problem is as follows,
when it drives there is a rattle that lasts for about 1 second when the speedo reaches 20 km p/h in 1st gear, 2nd gear 40 km p/h, 3rd gear 60 km p/h, 4th gear 80 km p/h and 5th 100 km p/h. There is no rattling noise from the engine compartment 5 minutes into the first drive of the day, it also does not occur when the accelerator is pressed slowly and speed slowly increases. I think it could be the through out bearing in the clutch assembly but any other "easier" troubleshooting guidance would be greatly appreciated.