Engine chatter?

Just dropped a new motor in my 68 camaro. When i fire up the car there is a chattering sound coming from the back of the motor towards the firewall may be underneath the car. Chatters at idle and in drive, doesn't seem to be affected by RPM. I have solid motor mounts and a poly tranny mount. Chatter does not sound like incorrect valve lash. Does not sound like detenation, car runs only on 110. When we first fired up the car we didn't hear any chatter but may have been due to open headers. Once I hooked the exhaust up is when we heard it. Doesn't start chattering initially when I fired up the car but after a few seconds it starts. Any suggestions would be a great help. Thanks