ya i would go with what he said - an check the all the mounts for things like rot in the rubber , being bent, cracked things of that origin... an if u dont see anything grab the top of the engine an try shakin while its off too see if it moves much, then start it up an get out open the hood - gas it a bit while ur watching it ,pull the arm on the trottle body a bit an see if the it shakes bad or abnormal - if so ya prob. got a bad one but its not being visible - But - if not an everything looks cool - then i would have someone check over the entire exhaust system from the header back an check for small or big cracks - holes an what not.... ya never know - my boss had a wierd noise on his truck an he said he just put a new muffler an cat on it so i was curious - i looked it all up an down - i come to find he has 2 serious cracks on the bottem neck of his headers both in the same spot he musta hit a bad bump an flexed the whole body alot an "snap" - it can happen really easy in the cold before they are warmed up good - so i changed both headers on it bolted the down pipes on an started it up - he said "damn" my truck was hurtin for some headers bad huh? - i laughed an said yep - u can add that to this weeks pay check ....
Good Luck!
hope ya find ur problem!
