99 Taco 3.4L Vibration at stoplight

Hi there, On my 99 TRD Taco, I've recently noticed that after warming up and running without any noises and rattles there is a slight vibration (felt in the floorboards) while at a stop light, sometimes it's more or less severe. When I put it in park it goes away, which tends to hint it may be something loose or rattling once the rpm are around 600rpm in drive and when it park at 650-700rpm it goes away.

I tightened all the bolts I could access underneath, the steering had one loose bolt but that's it. The shields underneath are all tight and the joints are all lubed up and looking good.

Any suggestions where it could be coming from?? BTW how do you adjust the idle on these engines? I don't see the usual screw on the intake as in the

Thanks