Rumble noise when car in gear and foot on brake at a red light

A rumbling sound when you are at a traffic light is likely to be in the exhaust system. Hopefully not the Catalytic Converter breaking up. Get a friend to sit in the truck and apply the hand brake and with the engine running get them to put their foot on the brake, do you hear the noise standing outside the truck? Don't get under the truck but can you locate the general area the noise is coming from. Get an exhaust shop to look at it, it may be as simple as a broken exhaust mount ( a rubber hanger costing $6 or so).