Did you ever sort out the "sewing machine" noise problem?
I have the same noise and I'm currently attempting diagnosis.
Like you, the noise is only when hot and the sound is as you describe (good description incidentally!) And comes from top of the motor, drivers side.
I bought this at 140,000. It already had the cam belt, idlers, W-pump, seals all done @ 95,000 miles by the previous owners garage.
I also got the (Dr.) Harbor Freight - stethoscope and got to work listening.
After removing the cam belt top plastic cover, the noisiest place was the top idler bearing - replaced it - no apparent change.
Took off all the ancillary parts - AC compressor, P-Steering pump, alternator and fan and bracket. W-pump is quiet (Aisin brand) but bottom tensioner pulley was noisy - replaced it - no apparent change.
Took off both valve covers and checked shim/camshaft clearances - 1 x exhaust @ 15mil and 2 x 14.5mil otherwise all others in spec.
Scissor gear(s) tension appears good and same as passenger side gears.
Looked down intake ports and could see intake valves and cylinder honing lines when valves open. All looks good and no foreign objects (like a bolt etc.) Rattling around on top of the valve faces.
Am now going to remove camshafts from the drivers side, attempt to lift out 'buckets' and inspect valve springs etc.
A thing I noticed is how clean the drivers side valve-train is compared to the passenger side. A lot more carbon there, maybe 'cos PCV valve 'suck' side?
Monday, November 24th, 2014 AT 1:26 PM