need help with a tamiya

[jamie 6.5t]

right i swapped my madrat for a tamiya tb-02 chasis and shells and have dropped a fusion exceed 6.5 brushless kit into the chasis with a 4500nimh battery and was just wondering if you know why my car keeps on making a chrunching sound when it gets to half revs and when i get past half revs it goes back to being quite again it keeps on doing it and is starting to worry me :/ the four wheel drive aspect is supperb best ive had only i cant get further than about 5 metres due to my receiver antena has snapped off need to get a new one quick time just please advise me about the chruching noise please