Thanks, Deathrat. Hey, you seem to be the guy I need to talk to. Does this sound familiar for a '96:
On gradual accelleration, between 1800-2000 rpm (45-50 mph or so) I get this rumbling sound. Almost sounds/feels like slipping between gears. Also happens during cruise mode, same rpm, but cruising around 65mph. Give it the gas and the rumbling stops.
They've checked the tranny and igntion and it checks out okay....nobody can figure out what'* wrong (unless this is normal for mid-90'* bonnevilles). On a related note, I was having trouble with this broken tube under the upper intake manifold. It was leaking coolant. The more the coolant leaked (before we found out what the problem was with the coolant), the worse the rumbling became. After we fixed the problem under the manifold, the rumbling remained, but consistently under the conditions like I described above. This is why I'm changing the plugs...hoping it'* some kind of ignition thing.
Whattayathink? Sound familiar at all?