Switching oil is not going to help and for the year of your Acura I would guess it's your motor mounts. Motor mounts are what holds the motor to the frame of the vehicle; its a tough rubbler that wares and breaks down over time and can cause that brief clunk feeling noticed in reverse, can also be noticed when put into drive. IF that's what it is, your safe to drive it but it will get worse as the rubbler wares down and the engine shifts in the frame as you drive. (Not really good for the car.) Stop in at any of your local Acura Dealers and have them take a look at it. -Just make sure to call ahead and request a tech to test drive it with you. Tell them you hear a strange noise from your engine and your not sure what it is. (Saves time & they will normally tell you to come in right away) Once your sitting with the tech tell him what you actually heard, and make sure you can reproduce it. If it's your transmission they will be able to tell you that; if its the motormounts, they will be able to put it on the rack and SHOW you it. But from what your describing, my GUESS would be it's the Motor mounts. Play it safe and get it checked ASAP. If it ends up being your trans then you could be stuck sitting in the middle of the freeway.