It could be a leak in the exhaust manifold. It would be more of a tappet sound and mine goes away when it warms up. Also check all your leads, Dissy cap and rotor. You may be losing a cylinder under load. Being a low compression engine it runs fine a cylinder down but might give a pinging feel under load.
My Land Rover has made me a better Mechanic........
D2 TD5 Manual, CDL, Detroit Rear, Truetac Front, 2" OME lift, 32" MT Tyres, ARB Bull Bar, Winch, TJM Roof Rack, Driving Lights, Dual Batteries, ARB Side Steps, Rear Draws, Uhf.