This winter I've noticed that there seems to be some valve train noise on starting the engine when it's below 40 degrees or so. The noise isn't very loud, more like a soft thud-thud-thud than a clicking sound, and only goes away when the engine fully warms up--3-4 minutes after starting. Oil pressure looks OK (it's an '05 so it has a functioning gauge), using a Nissan filter and 5W30 oil (dealer does the changes; yes, I'm lazy). Oil level is normal. The engine seems to be running fine while it's making the noise, and on warming up the noise is completely gone. The noise has persisted through several oil changes and, as I said before, doesn't happen if the temperature is above 40 or so. Is this happening to others? Something to worry about or just normal? Thanks for your help!