Hello, I have a 97 Bonneville SE with the 3800 Series II N/A motor with just shy of 250k. On cold starts I have a rattle that lasts for maybe half a second. It doesn't sound mechanical in nature (like a rod knock or anything), instead it sounds more like either pre-detonation or some sort of chatter potentially coming from the belt system. It ONLY happens on a cold start (the car has been sitting for more than 2-ish hours). I believe I may be having some sort of ICM failure causing to an intermittent P0300 code. You can feel the car misfire every 10 or so seconds going at speeds above 40 once at operating temperature, and occasionally bucks hard from misfiring when under heavy throttle (such as getting on the highway). I'm beginning to wonder if the ICM is going out and causing a failure to advance the ignition properly, and if that is the case, if that would also cause this rattle on cold start since you would want more ignition advance on a cold engine? No other codes other than a P0300, so I doubt it'* coil, O2 sensor, MAF sensor, etc related cuz those would all trip other or more specific codes (I.E. a coil pack going bad would cause a P0301/P0304, P0302/P0305, or P0303/P0306 codes in respective pairs since each coil controls two cylinders). I am mechanically inclined, outside of advanced electrical component diagnoses / knowledge, and have dealt with both rod knock and lifter tick before and this definitely does not sound like either.

At 250,000 it might be a little bit of lifter clatter.

Let'* call this a warm start.

Does it do this when it is ice cold, like when it has been parked twelve hours?

How many miles on the spark plugs?

How many miles on the wires?

What is the fuel pressure ignition on engine stopped?

What is the fuel pressure engine idling?

What is the fuel pressure engine idling with vacuum line removed from fuel pressure regulator and capped off?

How many miles have you owned the car?

I think it is premature to condemn the ICM based on what I know of it so far. Also, it'* really hard to have detonation on a cold engine with low cylinder pressures and the throttle plate closed and no load on the engine and worn out properly-gapped spark plugs.

I agree, odds are low here.

Could be all lifters drained down and taking that half-second to pump up. Maybe.