Need help with starter problem
 

Hi everyone,
Ill get straight to the point, I just swapped a C4 for a T5 on my 68 mustang, kept the original starter, and when I start the engine now, it makes a horrible grinding sound for the duration of the starting sequence then it goes away once the engine is running. It sounds like that noise you hear when you turn the key on an already running engine. After this first happened, I bought and installed a brand new 90s starter and it made the same noise so I went back to my original starter until I can figure out what on earth is going on. Does this sound familiar to anyone??
Thanks for your time!!
Frank :)