Gas and Oil Home Heating Furnaces - blower fan relay chatter on low speed

Hello to all and thanks in advance for any replies.
I have an old (20 year) Sears natural gas fired attic furnace with outside regualar A/C unit. Don't use the heat a lot here in Houston, but it got cold last night!
The problem is when the inside blower fan tries to start in low speed. Then the start relay "chatters" or bounces for maybe 4-5 seconds and makes a lot of noise. After the few seconds, it settles down and runs normally. Next cycle - same thing. I am somewhat knowledgable in HVAC and am an electronic tech. so I know how to not hurt myself at least. I jumped each cutout and limit switch to troubleshoot and got down to the fact that the relay chatters with all bypassed - (don't worry, I put all back to normal). So thinking the relay was going bad, I replaced it and also the start capacitor since it is an older unit. SAME THING! Can the motor be going bad? Maybe just dragging? Bleeder resitor? nay ideas on how to check these or any other suggestions are most welcome.

With the T-stat turned off, and then set the fan switch to "ON".

The fan will kick in high speed and see if the noise comes on.

if the noise comes on, then turn off the power, and remove the blower door.

reach in and turn the blower cage by hand to see if the noise is coming from the blower cage. it maybe is lose, and hitting on the housing of theblower cage.

I would put the meter on the control wire and watch what it does. You may have a bad connection from wherever it gets its control voltage from...like maybe the circuit board.

Thanks to both of you! I'll try the suggestions. The noise is defiinately coming from the relay chattering and when you turn the fan on high it does not happen. Just when the "low side" of the motor contacts try to make. I'll check control voltage and let you know.

You said attic furnace . Is this a lay down unit? If so some have a blower control on them for time on and temp off. This should go to a spdt relay that turns high or low on for you at the blower and points are burned in it.;) ED

Turns out the blower motor is bad. It has 4 speeds as far as windings go. It was wired for low for heat and high for cool. There is also a M-Low and M-High, each faster than low. Trying the M-low does help but relay still chatters at first. The transformer voltage si fine in all modes. This unit has a DPDT relay which does engage in either heat or cool mode. The relay just selects the windings to use and the low speed connection is wired through the limit and safety switch. Checking current the motor is pulling too much and the blower cage is free to turn -- so a new motor is next.
Thanks to all for the help.

Make sure you get the same RPM and it turns the same way CW or CCW.;) ED