Ignition warning light on 87 5kcst

["Smeins, Larry" <lsmeins@ball.com>]

This past week the warning light that looks like an engine with a lightning
bolt has been intermittently flashing at me in my 87 5kcst.  The manual says
this means the engine is pinging and timing can't be retarded enough to stop
it.  The car was just released from the shop where  a clutch and timing belt
were replaced.  It has been sitting for several weeks and got a fresh tank
of fuel when I picked it up.  The Audi has 240k miles on the odo.  My guess
at possibilities include: 1. bad tank of fuel, 2. lifters intermittently
clattering enough to fool knock sensor, 3. Tighter timing belt changed
static timing a bit, 3. timing belt 1 tooth off on distributor/ oil pump
drive shaft (not sure if this applies to the I5).  I would appreciate any
thoughts on where to start looking.  I prefer starting with the
easiest/cheapest possibility or the most likely and need some expert/
experienced opinion.

Thanks in advance,
Larry