Milly, A drivetrain clunk can be anything from bad motor mounts, transmission mount, rear axle carrier mount (inside trunk), exhaust making contact to the body, driveshaft joints, transmission output shaft nut loose, etc. Have seen it all. Best way to approach this is have a helper manipulate the clutch while in gear with the parking brake on and wheels chalked. Just enough to load the components without stalling the car. sometimes second or third gear works best. Then while they are doing this go around to the various components to try and isolate the noise.
Does it make the noise in neutral or just when leaving from a stop?