Engine noise when standing in drive or in reverse

My Mazda 626, 2 L., 2001 recently started making noise from the engine area when at a standstill and the transmission is engaged in "drive" or when driving in reverse. The noise also becomes apparent just before braking to a stop. Occasionally (maybe once every 5 or 10 minutes) I can feel a small "skip" in the engine. Engine mounts were checked recently and one was found to be cracked and engine mount was replaced . When the problem did not seem to be alleviated a cracked spark plug was found and all plugs were replaced. Any suggestions as to the cause of the problem?