I'd bet money its piston rattle, I've seen a bunch of them that do it. On the 103 the piston skirt is long enough it protrudes about 10-12mm below the bottom of the bore. The 104 piston's skirt is short enough it doesn't protrude below the bottom of the bore at all. That short skirt allows the 104 piston to rock in the bore especially when the engine is cold and the clearances are larger. I wouldn't worry about it, unless you plan to run it near the redline a lot when it is cold.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff