I have a ruud furnace and it won't turn on when set on "auto" mode,its only turning on in "manual" mode and blows heat.The fan turns on when put in the "on"/"fan" mode. I have to turn it off manually once the rooms have reached the desired temp.I could here the blowers start than they shut off automatically after a minute or two. There is heat coming out of the egsaust vent on the side of the unit and I'm hearing a noise comeing from the area near the blowers.

The fan contol that operates in the heat mode is defective or out of range. By switching to the on mode you are using a relay in the unit to bypass this control and bring the unit on manually. Find the defective part and replace it. Since there are many different type of these controls you will need to use the wiring diagram on the back side of the units blower motor door access panel to find the one in your unit.

BTW you can leave the blower run all the time until you find the defective part since it will not hurt anything as long as the motor operates properly.

NOTE open all registers in the home and make sure the air filter is clean and the return air is no obstructed.

Not really sure whear to start. Would I need to shut off the breaker or keep the power on and would I be able to "see" the defected part

PLEASE keep you posts on the same thread.

You start by turning off your power to the unit. Then you can use a meter to find the offending part. Eye sight will not work in finding the defective part you need to use a meter and test the circuits. Follow the wiring diagram on the back side of the blower motor access panel for starters. I cannot do the testing from here.