Toyota Repair: 97 RAV4 4WD, engine failure, engine noise

Question
I'm considering purchasing above vehicle, w/148000 miles.  But, I hear a knock in the engine.  It happens when first started but goes away.  I can also get it to do it briefly when revved between 2000-3500RPM, then it goes away.  It's not real loud & sounds like it's coming from the valve cover area.  Are engine knocks always a sign of engine failure; or is there any reason it would be normal or something less serious under some conditions, and how could I tell?  Check engine light is NOT on. Second question, only if you don't sense a pending engine failure -- the remote fuel release doesn't work & it's down to 1/4 tank of gas.  Any way to easily open that manually to refuel once or twice before fixing it?

Answer
I would advise not to buy this car because of the engine noise, it sounds like it may be a bearing noise or piston noise, not good news, save your money and look for another car that at least has no noises, buying a used car alsways has certain risks, it may be a good idea to pay an inspection fee and have a technician check the car over before you purchase it, it may just save you a lot of grief later.