I have a 2007 Toyota Camry with about 150,000 miles. Hear a small knock.

always check colant levels and never just go and buy new replacement parts unless completely necassary. ay shop will tell you it's neccassary so they can make more money on selling your old prt that most likely still works just finme,, just needs to b cleaned properly.. alot of hack shops will use carb cleaner (acetone : plastic eater) on electrical part's like the idle air control valve which will destroy it's sensor over time. Engine Cycle Timing Problems The engine cycle's rhythm can get disturbed if the spark plug firing and the fuel mixture combustion are not synchronized. In the event that a spark plug timing and combustion don't match, there is bound to be a knocking noise. This problem can only be dealt with by a car repair center. Mechanical Alignment Problems One of the prime reasons leading to knocking or clicking noise in engine, is problems with the alignment of some of the engine components, like, more than normal clearance in the crankshaft connecting rod bearings or it may be due to entirely defective crankshaft bearings. If this is actually the case, you can expect quite a loud noise, as metal hits metal surface, creating a ruckus. In this event too, you need to get to the car repair center as soon as possible. Too Lean Air Fuel Mixture Sometimes, knocking noise can be attributed to too lean a air-fuel mixture in the combustion chamber. This condition of lesser fuel with more oxygen availability, which may lead to pre-ignition and to the knocking noise ultimately. You need to get the engine tuned properly to prevent this from happening. One of the best ways to prevent this situation is to go for regular engine servicing. EGR Valve Problems If the exhaust gas recirculation (EGR) valve mechanism is messed up, it may result into an engine knocking noise, due to an abnormal change in the air fuel mixture ratio. EGR valve cleaning and repair might solve the problem quite easily. It is highly advisable that you get your car to the servicing center as soon as possible, if loud knocking noises present themselves. This will save you a lot of time and money spent in repairs later.

SEE A MECH TO DIAG ISSUE THEN YOU CAN DEAL WITH THE FACTS NOT NOISES