You might be just hearing air being pulled into the carb. 18, 500 by a grandma is almost as bad as 185,000 by a leadfoot. If that motor only saw city miles, there could be some carbon buildup internally. If the engine sat for a long time without being run regularly, there are other potential issues, like a valve sticking in its' guide, or stuck piston rings, or even corrosion on valve seats that can cause cylinder sealing issues. You're positive the odometer's correct? Also, one other note, don't always trust the rpm's shown on a (cheap?) tach-they can be inaccurate. 800 is a bit high, especially with an auto trans. Again, basics. Document changes as you go along, and go through things methodically. My last post is bare bones to getting something to run right. In response to your first question, the engine makes less suction, or vacuum, at higher rpm, and as rpm increases, the the mechanical defect usually is felt less exactly because the engine is turning faster. At least, that's what I think.
