Linear velocity drift robot_localization with imu

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Hi all,

I am using robot_localization to fuse odometry and imu measurements. While my robot is standing still, the estimated pose blows up towards the x and/or y direction and the velocity increases steadily. I believe this is because my IMU messages are reporting that there is acceleration in the x-y direction when there is not. The acceleration that is seen on the imu axes are from gravity because on my robot it is impossible to mount the imu perfectly level. The angle is accurately reflected in the orientation.

Odometry message:

header: 
  seq: 180
  stamp: 
    secs: 0
    nsecs: 0
  frame_id: odom
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: -4.57017288436e-23
      y: -1.37577341871e-27
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: 0.0
      w: 1.0
  covariance: [0.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.2]
twist: 
  twist: 
    linear: 
      x: 0.0
      y: 0.0
      z: 0.0
    angular: 
      x: 0.0
      y: 0.0
      z: 0.0
  covariance: [0.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.2]

IMU message:

header: 
  seq: 3565
  stamp: 
    secs: 1437400985
    nsecs: 945771932
  frame_id: base_imu_link
orientation: 
  x: 0.0115444314118
  y: 0.0118909824299
  z: 0.0816176668022
  w: 0.996525908899
orientation_covariance: [0.0025, 0.0, 0.0, 0.0, 0.0025, 0.0, 0.0, 0.0, 0.0025]
angular_velocity: 
  x: -0.07
  y: 0.0
  z: -0.0
angular_velocity_covariance: [0.02, 0.0, 0.0, 0.0, 0.02, 0.0, 0.0, 0.0, 0.02]
linear_acceleration: 
  x: -0.210292734375
  y: 0.2711971875
  z: 9.72747539062
linear_acceleration_covariance: [0.04, 0.0, 0.0, 0.0, 0.04, 0.0, 0.0, 0.0, 0.04]

robot_localization message

header: 
  seq: 14858
  stamp: 
    secs: 1437401408
    nsecs: 104844999
  frame_id: odom
child_frame_id: base_link
pose: 
  pose: 
    position: 
      x: 752.873872865
      y: -960.277044656
      z: 0.0
    orientation: 
      x: 0.0
      y: 0.0
      z: -0.0129782931114
      w: 0.999915778407
  covariance: [1235363.2798552709, 0.09966583512707781, 0.0, 0.0, 0.0, 0.00012219044159253388, 0.09966583492106752, 1235363.2253877113, 0.0, 0.0, 0.0, 8.345895540980487e-05, 0.0, 0.0, 4.997912991703062e-07, -1.6226536599895273e-14, -1.1710583642991047e-14, 0.0, 0.0, 0.0, -1.6226536599895264e-14, 4.995829440611683e-07, -9.105011207999492e-19, 0.0, 0.0, 0.0, -1.1710583642991045e-14, -9.10501120799949e-19, 4.995829440617729e-07, 0.0, 0 ...