Well I checked the MAF sensor and tried running without it for a short drive. No change. I gave it a clean and put it back and it was the same.
I checked the vacuum hoses and there are no obvious leaks.
The air filter isn’t particularly dirty, though I might change it anyway.
The fuel filter looked like it had never been changed and the screw holding the clip in place was rusted to oblivion. I removed the old fuel filter and put a new one in and the car seemed to run better for a while. The idle smoothed out too. I filled the tank and drove about 75 miles last night and it was very smooth, almost quieter than it was originally.
Parked it overnight and in the morning the low growl on accelerating was back. And while the idle is now consistent, there is more vibration than I remember. It holds steady at about 750RPM
The only thing I’m thinking now is some valve issue or fuel ignition coils, or injectors. If that was the case though, why was it so smooth last night?
Still no engine light though, so whatever it is is not regstering in the ECU. Maybe I’m just paranoid and it was always this way? I have been told that my model engine can be very loud when revving.